Clockwork Steed [Fifth Edition]

Clockwork Steed enters the battlefield with four +1/+0 counters on it. Clockwork Steed can't be blocked by artifact creatures. At end of combat, if Clockwork Steed attacked or blocked this combat, remove a +1/+0 counter from it. {X}, {T}: Put up to X +1/+0 counters on Clockwork Steed. This ability can't cause the total number of +1/+0 counters on Clockwork Steed to be greater than four. Activate this ability only during your upkeep.