Thistledown Duo is a common Creature — Kithkin Soldier Wizard card from Shadowmoor (SHM), collector number 152. Current market price: $0.09 (foil $0.40), TCGplayer market prices via Scryfall.
Whenever you cast a white spell, this creature gets +1/+1 until end of turn. Whenever you cast a blue spell, this creature gains flying until end of turn.