Whenever Genghis Frog or another Mutant you control enters, create a Mutagen token. (It's an artifact with ", , Sacrifice this token: Put a +1/+1 counter on target creature. Activate only as a sorcery.")
Genghis Frog turns Mutant creatures into a +1/+1 counter engine: every Mutant entering spawns a Mutagen token that later grows a creature. Flood the board with Mutants and tokens, bank Mutagens, then dump counters onto an evasive threat (or the trampling Frog himself) for a lethal swing, supported by counter-synergy payoffs.
Free, repeatable +1/+1 counter generation that snowballs with every Mutant ETB
Low mana value (2) makes the commander cheap to deploy and recast
Trample built in, so pumped creatures push damage through chump blockers
Slots cleanly into proliferate and counter-matters shells in strong GU colors
Mutant is a tiny creature type, so true 'tribal' support is thin and you lean on counter payoffs instead
Mutagen counters are sorcery-speed and one-shot, making them slow and removal-vulnerable
Heavily board-centric; a single wrath sets the whole engine back
Counters do nothing without evasion or a way to break through go-wide defenses
Legendary Creature — Frog Mutant Rogue
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, Sacrifice this token: Put a +1/+1 counter on target creature. Activate only as a sorcery.")