Desert Were-Worm is a rare Creature — Dragon Wurm card from The Hobbit (HOB), collector number 225. Current market price: $1.16 (foil $1.64), TCGplayer market prices via Scryfall.
This creature gets +2/+0 for each Mountain you control. Whenever you attack with creatures with total power 12 or greater for the first time each turn, untap all attacking creatures. After this phase, there is an additional combat phase.