Ornery Tumblewagg is a rare Creature — Brushwagg Mount card from Outlaws of Thunder Junction Promos (POTJ), collector number 171p. Current market price: $0.53 (foil $0.57), TCGplayer market prices via Scryfall.
At the beginning of combat on your turn, put a +1/+1 counter on target creature. Whenever this creature attacks while saddled, double the number of +1/+1 counters on target creature. Saddle 2 (Tap any number of other creatures you control with total power 2 or more: This Mount becomes saddled until end of turn. Saddle only as a sorcery.)