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.

Masters Edition II

Clockwork Steed comes into play 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. Play this ability only during your upkeep.

Fifth Edition

Clockwork Steed cannot be blocked by artifact creatures.
When Clockwork Steed comes into play, put four +1/+0 counters on it. At the end of any combat in which Clockwork Steed attacked or blocked, remove one of these counters.
o
X, oc
T: Put X +1/+0 counters on Clockwork Steed. You may have no more than four of these counters on Clockwork Steed. Use this ability only during your upkeep.

Homelands

Cannot be blocked by artifact creatures.
When Clockwork Steed comes into play, put four +1/+0 counters on it. At the end of any combat in which Clockwork Steed attacked or blocked, remove one of these counters.
o
X, oc
T: Put X +1/+0 counters on Clockwork Steed. You may have no more than four of these counters on Clockwork Steed. Use this ability only during your upkeep.

Can attack or block even if it has no counters.

Loses a counter even if it is affected by a Fog-like effect which prevents it from dealing damage.

If Clockwork Steed has four or fewer +1/+0 counters on it when its last ability resolves, it can wind up with a maximum of four such counters on it. If it has four or more +1/+0 counters on it, the ability will have no effect.

When Clockwork Steed's last ability resolves, you can choose to put fewer than X +1/+0 counters on it.