Pollywog Symbiote 
Creature — Frog
Each creature spell you cast costs less to cast if it has mutate.
Whenever you cast a creature spell, if it has mutate, draw a card, then discard a card.
Legality
standardNOT LEGALpioneerLEGALmodernLEGALlegacyLEGALvintageLEGALcommanderLEGAL