Stampeding Proudhorn

Cumulative upkeep {g} (At the beginning of your upkeep, put an age counter on this permanent, then sacrifice it unless you pay its upkeep cost for each age counter on it.)
Whenever Stampeding Proudhorn attacks, search your library for a creature card with converted
mana cost equal to the number of age counters on it. Put that creature card onto the battlefield
tapped and attacking, then shuffle your library.