Riverfall Mimic is a common Creature — Shapeshifter card from Eventide (EVE), collector number 111. Current market price: $0.11 (foil $0.52), TCGplayer market prices via Scryfall.
Whenever you cast a spell that's both blue and red, this creature has base power and toughness 3/3 until end of turn and can't be blocked this turn.