Cryptic Trilobite 

Creature — Trilobite
This creature enters with X +1/+1 counters on it.
Remove a +1/+1 counter from this creature: Add . Spend this mana only to activate abilities.
,
: Put a +1/+1 counter on this creature.
Legality
standardNOT LEGALpioneerNOT LEGALmodernNOT LEGALlegacyLEGALvintageLEGALcommanderLEGAL