Bulwark Ox is a rare Creature — Ox Mount card from Aetherdrift Promos (PDFT), collector number 7p. Current market price: $0.58 (foil $0.88), TCGplayer market prices via Scryfall.
Whenever this creature attacks while saddled, put a +1/+1 counter on target creature. Sacrifice this creature: Creatures you control with counters on them gain hexproof and indestructible until end of turn. Saddle 1 (Tap any number of other creatures you control with total power 1 or more: This Mount becomes saddled until end of turn. Saddle only as a sorcery.)