Cobalts are large reptilian great beasts found mostly in the wilderness of Vabbi. They come in two different breeds. The warrior types resemble Sand Drakes and walk on four legs, while the paragon and mesmer types are biped and have disproportionately large heads that resemble aliens in fiction such as in the movie Alien or the Zerg in StarCraft.
The name Cobalt is likely a reference to the metal Cobalt. Cobalt is used to make a pigment known as Cobalt blue and Cobalt Mokele and Shriekers have a blue coloration on the back of their heads and necks, while Cobalt Scabaras are completely blue.
For a list of all Cobalts in the game see: Category:Cobalt.