Slith

Slith is a creature type unique to the plane of Mirrodin. They are depicted as humanoids with large skull-like heads, formed from rock and/or metal.

Description
There were five Slith in Mirrodin. Each of these uncommon 1/1 creatures had a mana cost containing two colored mana of the same type and the ability "Whenever this creature deals combat damage to a player, put a +1/+1 counter on it".
 * Slith Ascendant (Flying)
 * Slith Strider (Card draw)
 * Slith Bloodletter (Regenerate)
 * Slith Firewalker (Haste)
 * Slith Predator (Trample)

Darksteel added a modular artifact Slith.
 * Arcbound Slith

Storyline
Like almost all beings on Mirrodin, slith are not native to the plane but were brought there against their will.

The slith incubate in the Great Furnace's heat, emerging on Mirrodin's surface only when the suns have aligned overhead. Born amid molten metal, the slith have more than adapted to the perils of a metal world. A slith's form and function are determined by the color of the sun under which it's born. Instinctively drawn to the light of its "mother-sun," each slith follows that sun's path around Mirrodin. Goblins fear the slith, believing they are children banished from the womb of the Steel Mother, deep within Kuldotha.

Trivia

 * The design of the Slith cards was based on Whirling Dervish.
 * Tempered Sliver gives all Slivers the Slith combat ability, and has art reminiscent of Slith Predator</c>.
 * The Slith mechanic and a variant thereof were considered a knight mechanics in Throne of Eldraine.