Decapod Aberrant

Devolve (Whenever a creature enters the battlefield, if that creature's power and/or toughness is less than this creature's, put a -1/-1 counter on this creature.)
Whenever a counter is removed from a permanent or player, put a +1/+1 counter on Decapod Aberrant.
Remove three counters from Decapod Aberrant: Add {g} to your mana pool.