Clockwork Steed

This creature enters with four +1/+0 counters on it.

This creature can't be blocked by artifact creatures.

At end of combat, if this creature attacked or blocked this combat, remove a +1/+0 counter from it.

X, T: Put up to X +1/+0 counters on this creature. This ability can't cause the total number of +1/+0 counters on this creature to be greater than four. Activate only during your upkeep.