Amoebus Youngling

Changeling
When Amoebus Youngling enters the battlefield, choose a creature. Put age counters on Amoebus Youngling equal to its mana value.
At the beginning of your upkeep, remove an age
counter from Amoebus Youngling. As long as
Amoebus Younging has no age counters on it, it
is a copy of the chosen creature.