You may be looking for:
During Kerrigan's resurgence, Abathur discovered the creeper strain in an ash worm. After collecting the ash worm's essence, the creeper was formed. The ash worm is able to secrete enzymes to break apart the soil for fast travel. The creeper instead uses creep as a substitute for the enzymes to produce a similar effect.
Creepers are better defensively than the carrion strain due to their ability to rapidly reposition themselves for base defense. Thanks to their creep generation, they can also be very useful when paired with swarm queens, setting up forward creep positions with creep tumors. Using this combination can let the creepers fall back to quickly defend a base, then return to the front after they finish dealing with the threat.