This section details the uses and synergies (that I'm aware of) of most cards in the deck to elucidate the reasoning behind including these cards.
Angel's Trumpet: This encourages our hippo recipient to attack with the hippos. It also damages them for every hippo given to them during their turn. Has synergy with Verity Circle, Keep Watch, Borrowing 100,000 Arrows, Theft of Dreams, Arachnogenesis, Flurry of Wings, Nature's Revolt. Note: be careful when playing a sorcery-speed mass token generator like Clone Legion while the trumpet is out, it might kill you (as I've learned the hard way)! This issue is avoided by having a soul sister (e.g. Soul Warden) in play.
Arachnogenesis: One of our wincons, used when our friend attacks us with the many hippos we gave them. Has synergy with Primal Vigor, Coat of Arms, Mirrorweave, Angel's Trumpet, Bident of Thassa, soul sisters.
Arcane Denial: A counterspell befitting our "hug deck" disguise. Has synergy with Psychic Possession, Mind's Eye, Smothering Tithe.
Authority of the Consuls: Great card on its own, extra nice when giving out hippos. Has synergy with Verity Circle, Borrowing 100,000 Arrows, Theft of Dreams, Nature's Revolt.
Beast Within: Decent removal in any deck. Super Hippo really likes this one because it maintains or increases the number of creatures under our opponents' control, which is beneficial for our game plan.
Beck / Call: Possibly the best draw card in the deck. Usually we only play the Beck half, but fusing it can be useful in some situations, such as when we have a Bident of Thassa or need blockers.
Bident of Thassa: Believe it or not, this one is actually in here for the tap ability! Forcing our friend's hippos to swing is often a benefit for us, even if we're the one being attacked. The draw effect is nice too since we can give our general flying and consistently nab an extra card each round by swinging. Has synergy with Keep Watch, Borrowing 100,000 Arrows, Theft of Dreams, Arachnogenesis, Flurry of Wings.
Biomantic Mastery: Big draw spell that is best played after we have given out many hippos.
Birds of Paradise: Classic mana birbs that also serve as a combo piece alongside Intruder Alarm and Super Hippo. The birds are nice in opening hand as they will let us get our general out on turn 3.
Borrowing 100,000 Arrows: While situational, this can net us a ton of draw alongside Angel's Trumpet, Authority of the Consuls, Nature's Revolt, or Bident of Thassa. If we politic effectively, we can still get a ton of value out of this even without forced taps; "I'll give you hippos on the condition that you attack with all of them."
Chromatic Lantern: Allows us to make with any land, helping to maximize the number of hippos we can gift.
Clone Legion: One of our wincons. Has synergy with Primal Vigor, Coat of Arms, Mirrorweave, Nacatl War-Pride, soul sisters.
Coat of Arms: This is meant to be played following a Reins of Power, Nacatl War-Pride, Arachnogenesis, Clone Legion, Deploy to the Front, or Flurry of Wings, likely resulting in our victory. Coat could also be played early if we want to give a boost to our friend-opponent to help them eliminate our non-friend-opponents.
Copy Enchantment: It's always nice to have a second Mirari's Wake, Zendikar Resurgent, Primal Vigor, Privileged Position, Frontier Siege, Psychic Possession, or Smothering Tithe.
Curse of the Swine: Creature removal befitting our "hug deck" disguise. The fact that it exiles and we choose which dudes it hits makes this card totally underrated. Also, this card maintains the number of creatures under our opponents' control, which is beneficial for our game plan.
Curse of Verbosity: Draws us cards and steers attacks away from us.
Deploy to the Front: One of our wincons. Has synergy with Primal Vigor, Coat of Arms, Mirrorweave, Nacatl War-Pride, soul sisters.
Dingus Staff: Worth it for the dingus jokes alone. Becomes a deadly deterrent against our opponents' board wipes and sac outlets. Has synergy with Ezuri's Predation. Note: We must be careful when handling the dingus staff, as we run the risk of chafing ourselves pretty badly if the board is wiped while we have a multitude of tokens.
Dowsing Dagger
: This serves two purposes here - it gives an opponent a couple harmless creatures, and flips into a land that increases our hippo output by +3 per round. It is easy to flip, just equip to Super Hippo and give her flying via her ability if needed.
Essence Warden: Hey soul sister, ain't that baby hippo on the battlefield, life gain trigger, this life total ain't fair, you know! Hey soul sister, I don't want to miss a single trigger you do, tonight.
Ezuri's Predation: Very powerful wincon. Best used after having given many hippos! Has synergy with Dingus Staff, Primal Vigor, Coat of Arms, soul sisters.
Fellwar Stone: 2-cmc ramp cards such as this allow us to play Super Hippo on turn 3.
Flurry of Wings: One of our wincons, usually used when our friend attacks us with the many hippos we gave them. Has synergy with Primal Vigor, Coat of Arms, Mirrorweave, Angel's Trumpet, Bident of Thassa, Nacatl War-Pride, soul sisters.
Forbidden Orchard: A must for this type of hippo deck. We tap this thing every round, even if we're not using the mana for anything. The more tokens our opponent-friend has, the better.
Frontier Siege: Always choose Khans mode. This allows us to gift 4 more hippos every round.
Generous Gift: Decent removal in any deck. Super Hippo really likes this one because it maintains or increases the number of creatures under our opponents' control, which is beneficial for our game plan.
Gideon, Champion of Justice: One of our wincons. Best to drop him when we have a protective card like Arachnogenesis in hand, or Magosi, the Waterveil with a storage counter for back-to-back turns, as he is an instant target.
Gilded Lotus: Allows us to make more green mana, resulting in more gifted hippos.
Hall of Heliod's Generosity: This comes in handy since many of our enchantments wear big bullseyes. Useful for reclaiming combo pieces like Intruder Alarm or Nature's Revolt.
Intruder Alarm: Vital piece for our wombo combos (see combos section above for details). Can also be used in conjunction with Nature's Revolt to either give us infinite mayonnaise or keep everyone's lands tapped down.
Keep Watch: While situational, this can net us a ton of draw alongside Angel's Trumpet, Bident of Thassa, or Nactl War-Pride. If we politic effectively, we can still get a ton of value out of this even without forced attacks; "I'll give you hippos on the condition that you attack with all of them."
Lumbering Falls: When animated, this acts as a combo piece alongside Intruder Alarm and Super Hippo. Also, the inclusion of slow lands adds to our nonthreatening guise.
Magosi, the Waterveil: A combo piece (see combos section above for details). Magosi could also help to secure a victory alongside the likes of Gideon, Champion of Justice, Deploy to the Front, Nacatl War-Pride, Clone Legion, etc. The play here is to skip a turn to store a counter, then when it is our turn again we play a mass token generator, then activate Magosi for the extra turn. Taking back-to-back turns allows us to attack with our token army with less risk of interference since our opponents will not have the opportunity to untap or play sorceries.
Mana Reflection: This allows us to make more mana, resulting in more gifted hippos.
Mantle of Leadership: Toss this on Super Hippo, give out some tiny potamuses, and swing for commander damage!
Mercy Killing: Decent creature removal that gets around indestructibility. Super Hippo really likes this one because it increases the number of creatures under our opponents' control, which is beneficial for our game plan. This can be downright devastating in conjunction with Angel's Trumpet or Dingus Staff.
Mirari's Wake: Good in any deck, this doubles our hippo output and makes our army bigger.
Mirrorweave: This may seem like an odd choice, but it has worked wonders. Can be used offensively (similar to a Coat of Arms, turn all our tokens into the best creature on board) or defensively (turn all attackers into 1/1 hippos).
Nacatl War-Pride: One of our more interesting wincons. Per the oracle rules, we can actually have the copies attack players other than the defending player of the non-token Nacatl. For example, we can swing OG Nacatl at our friend-opponent who has 10 hippos, but we can send the 10 Nacatl copies at our non-friend-opponents. Has synergy with Primal Vigor, Coat of Arms, Mirrorweave, Dingus Staff, Keep Watch, Clone Legion, Deploy to the Front, Flurry of Wings, soul sisters. Also has some interesting synergy with Teferi's Protection in that we can cast TP in response to Nacatl's delayed trigger that would normally exile the cat tokens, but the copies cant be exiled if they're phased out, so we keep all the cats!
Nature's Lore: 2-cmc ramp cards such as this allow us to play Super Hippo on turn 3.
Nature's Revolt: This card puts in a ton of work in this deck. It is a board wipe deterrent and a combo piece (see combos section above for details). We could even use this to beat face with our lands, especially with Mirari's Wake, Mirrorweave, or following a Boundless Realms. Has synergy with Verity Circle, Intruder Alarm, Theft of Dreams, Borrowing 100,000 Arrows.
Open the Vaults: This comes in handy since many of our enchantments wear big bullseyes. Vaults can sometimes allow us to instantly set up a combo if we were hit with a lot of discard/mill/boardwipes.
Primal Vigor: Doubles our hippo output. Also doubles our mass token generators like Nacatl War-Pride, Ezuri's Predation, Clone Legion, Deploy to the Front, Arachnogenesis, Flurry of Wings. This is one of the strongest cards in the deck.
Psychic Possession: Stick this on the opponent who you suspect will be drawing many cards. If the enchanted player is cooperative, we can also spam Super Hippo's ability for a Trade Secrets-esque effect. At worst this card keeps us from dying to mill, at best it will draw us a buttload of cards.
Recurring Insight: Decent draw on its own, we can also spam Super Hippo's ability before casting Insight for a Trade Secrets-esque effect.
Reins of Power: This card is this particular hippo deck's raison d'être. Reins turned the original iteration of this deck from a lowly kingmaker to a veritable game winner. Playing this card when we've one opponent left who has like 30 hippos is the best feeling. Has synergy with Coat of Arms and Mirrorweave.
Seedborn Muse: Mo mana mo hippos. Also makes it easier for us to keep a open in case Super Hippo needs to evacuate.
Selvala, Explorer Returned: Provides green mana for giving out more hippos, and aids in our "hug deck" mask. Also serves as a combo piece alongside our general and Intruder Alarm.
Soul's Attendant: Hey soul sister, ain't that baby hippo on the battlefield, life gain trigger, this life total ain't fair, you know! Hey soul sister, I don't want to miss a single trigger you do, tonight.
Stirring Wildwood: When animated, this acts as a combo piece alongside Intruder Alarm and Super Hippo. Also, the inclusion of slow lands adds to our nonthreatening guise.
Suture Priest: Beloved by hippo decks across the land, this is a one of our strongest wincons.
Swan Song: A counterspell befitting our "hug deck" disguise. It also adds a creature to the board which is typically good for us.
Talisman of Curiosity: 2-cmc ramp cards such as this allow us to play Super Hippo on turn 3.
Talisman of Unity: 2-cmc ramp cards such as this allow us to play Super Hippo on turn 3.
Teferi's Protection: A great defensive card on its own, this also serves as a combo piece (see combos section above for details). Has some interesting synergy with Nacatl War-Pride in that we can cast Teferi's Protection in response to the delayed trigger that would normally exile the cat tokens, allowing us to keep all the cats!
Theft of Dreams: While situational, this can net us a ton of draw alongside Angel's Trumpet, Authority of the Consuls, Nature's Revolt, or Bident of Thassa. If we politic effectively, we can still get a ton of value out of this even without forced taps; "I'll give you hippos on the condition that you attack with all of them."
Thunderstaff: Protects us from the hippos we gifted. The tap ability allows us to assist our friend-opponent in the killing of non-friend-opponents, or to pump our own token army.
Transmogrifying Wand: While not a great card in most decks, it shines here because it maintains the number of creatures under our opponents' control, which is beneficial for our game plan. And it's a flavor win amirite? It also acts as a rattlesnake if we keep at least one counter on it. "Attack me and I'll turn your best dude into an Ox."
Treetop Village: When animated, this acts as a combo piece alongside Intruder Alarm and Super Hippo. Also, the inclusion of slow lands adds to our nonthreatening guise.
Verity Circle: While situational, this can net us a ton of draw alongside Nature's Revolt, Angel's Trumpet, Authority of the Consuls, or Bident of Thassa.
Wilderness Reclamation: Allows us to gift more hippos. Also helps us keep a open in case Super Hippo needs to bounce.
Zendikar Resurgent: Good in any deck, this doubles our hippo output. If we run out of gas, this can act as a draw engine (albeit an extremely inefficient one) by bouncing and replaying Phelddagrif.