Cankers are living Swarm projectiles fired from the chest cavity of Carriers and the Hive Beast. They have the ability to home in on targets, exploding on contact, but can be shot before reaching their intended target.
Cankers seem to be the successor of the LocustNemacyst, although they are aimed more for ground targets than anti-air.
Cankers fired from a Carrier are smaller than those fired from the Hive Beast, and also spread a flame after exploding, similar to an Incendiary Grenade, that can quickly down a player if they don't roll away, and will cause continuous damage to fortifications. As such, Cankers should be destroyed before they can cause any damage.