Mutable Explorer is a rare Creature — Shapeshifter card from Lorwyn Eclipsed Promos (PECL), collector number 186p. Current market price: $1.70 (foil $4.72), TCGplayer market prices via Scryfall.
Changeling (This card is every creature type.) When this creature enters, create a tapped Mutavault token. (It's a land with "{T}: Add {C}" and "{1}: This token becomes a 2/2 creature with all creature types until end of turn. It's still a land.")