Donatello, the Brains is a mythic Legendary Creature — Mutant Ninja Turtle card from Teenage Mutant Ninja Turtles Eternal (TMC), collector number 2. Current market price: $1.11 (foil $0.51), TCGplayer market prices via Scryfall.
If one or more tokens would be created under your control, those tokens plus a Mutagen token are created instead. Partner—Character select (You can have two commanders if both have this ability.)