Ulgohr, The Horned One

Cumulative upkeep - Sacrifice a creature.

When Ulgohr, The Horned One enters the battlefield, you may sacrifice a nonlegendary nontoken creature you control, if you do, search your library for a creature with a mana cost of X
and put it on the battlefield, X is 1 plus the sacrificed creature's mana cost.

You can only cast creature spells.