The Sultai version I've come up with for Pauper Mutate. It seems pretty decent if a little on the slow side. If you're looking for something less consistent but a fair bit faster, here's the Bant list: https://tappedout.net/mtg-decks/bant-mutate-miaggro/?cb=1605314362
Utilize Arboreal Grazer with a bounce land if you open with it to get a decent ramp into a T2 Migratory Greathorn. Even if you're not the beatdown and looking to stabilize, an early 3/4 is a solid way to dodge a bolt and immediately getting to fetch a basic land helps play other cards. Two Growth Spiral work as a solid backup plan if your opening lands aren't quite as ideal.
The Migratory Greathorn, Dreamtail Heron, and Cavern Whisperer feel like the strongest effects to keep building around for a MIDRANGE build and I really feel like 12 is necessary to get the most bang for your buck. More is fine if you can make it work but as we're limited to only one common in each color for mutate right now, this is what we get.
Lead the Stampede and Winding Way are there to provide card advantage in addition to Dreamtail Heron turning mutate into cantrips. Winding Way works extra well paired with Migratory Greathorn pulling lands out of the way but you still whiff on occasion.
The rest of the deck is toolbox creatures. Mysterious Egg is our non-white Lagonna-Band Trailblazer with Essence Symbiote providing additional support as well as a decent lifegain buffer against aggro and burn decks. Boot Nipper is still in the early trial phase as I'm torn between it and Vault Skirge for main vs sideboard but in initial playtesting the ability to either make a huge lifelinker OR give it deathtouch and use it as a blocker while you attack with other large creatures has been quite obnoxious for opponents. Lastly, there's Crypt Rats which are already excellent against Elves and Aggro but also become absolutely terrifying if you use them as a mutate target. They also provide a backup plan in the event that you get locked out of combat.
As for the sideboard, it contains a few more utility creatures for certain matchups as well as some handy tech if you need it. Burglar Rat can come in in place of Boot Nipper against control decks to provide you an extra way to attack their hand. Cast Down can swap in for Growth Spiral in matchups where you need to target specific creatures. Moment's Peace can help against hyper aggressive decks where things turn into slug-fests. Vines of Vastwood give you both a conditional hexproof against removal as well as a pump if you need that last bit of damage to take someone out. Teferi's Time Twist provides you a flicker effect to break apart stacks of your creatures in response to either removal or as a way to suddenly go for wide rather than one big creature.
The only effect I'm not sure where to fit into the deck either main-board or sideboard is a creature that lets you pick up another creature or permanent (similar to Kor Skyfisher to provide extra mutate effects if you want to stack cards onto a different creature than your initial target.
