The Meld Hound is a rare Legendary Creature — Dog card from Unknown Event (UNK), collector number RZ06k. Current market price: $49.99, TCGplayer market prices via Scryfall.
Whenever you cast a creature spell, exile cards from the top of your library until you exile a creature card. It melds with that spell. You may give the combined creature spell a new name. (The combined creature spell has the abilities and creature types of both creatures, their combined powers and toughnesses, and so on.)