Clockwork creature

Clockwork creatures are usually imitations of organic lifeforms and need to be repeatedly wound up.

Clockwork ability
Mechanically, this is represented as the card entering the battlefield with a set amount of counters on them, which they progressively lose when participating in combat.

Brothers' War
Clockwork avians were airborne machines of war created during the Brothers' War. Urza created flocks of small clockwork birds to combat Mishra's dragon engines. They were able to tear the engines' skin and destroy them from the inside by igniting the internal fuel tanks used for the napalm-like fire breath.

One of the mechanical birds that survived was used centuries later by Ravidel to defend Castle Melmereth, and both killed and was destroyed by a Cockatrice.

Wizards' School on Ulgrotha
Clockwork gnomes were minute creations built by students beginning their education at Feroz's Wizards' School on Ulgrotha, used as servants on the Floating Isle.

Clockwork swarms are collections of these clockwork creatures. Originally built to drive vermin out of buildings, they frequently went awry: many of them were destroyed because they were too dangerous.

Steel Island
Steel Island, one of the Burning Isles was also inhabited by Clockwork Gnomes.