Coilfang Guardian can be found in Serpentshrine Cavern.
These mobs spawn as adds in the The Lurker Below encounter. When The Lurker Below dives, he spawns a total of 9 adds: 6 ranged Coilfang Ambushers (2 on each platform), and 3 melee Coilfang Guardians. All these adds can be Crowd controled.
The first two should be tanked by the Main Tank and Off Tank and the last one should be tanked by one of the DPS warriors/feral druids whose platform adds are dead. They need to be sheeped and killed one by one. When each pack of Ambushers dies, the group that's killing them has to switch to the Guardians.
Attacks and abilities
Coilfang Guardians - 70k Health. They have nasty cleave that can easily one shot clothies so make sure everyone but the tanks stays back.
Guardians must be killed before The Lurker emerges, or he will probably bug. Also note that if the guardians are not sheeped fast enough they will teleport to the platforms if one of the healers there gets aggro.