Items, NPCs, quests, objects and spells of patch 1.12.1, read live from the realm itself.

Crawler Mucus

NPC	Level	Zone	Chance
Pygmy Surf Crawler	5 - 6	Durotar	80.0%
Surf Crawler	7 - 8	Durotar	80.0%
Encrusted Surf Crawler	9 - 10	Durotar	80.0%
Corrupted Surf Crawler	10 - 11	Durotar	80.0%