Fossorial Bile Larva
From Lord Aj'qirai. If a bile larva stays burrowed for an extended period of time they begin to lose their pigment. When they finally do emerge, they can be quite rave... Beast type. i just got this pet from skinning a scorchwing felspitter. i had fiendish leather rank 2.
The Fossorial Bile Larva is a battle pet obtained through skinning on Argus, requiring Legion Skinning to have a chance at the drop.
Guide awaiting review
Learn Legion Skinning First
Before farming, obtain 
Skinning Technique: Legion Gutting by skinning creatures across the Broken Isles until a quest drop prompts you to collect 10 stonehide leather. Completing that quest trains you in Legion Skinning, which appears to be a requirement for the Fossorial Bile Larva to drop at all — one farmer had 800 skins with Cataclysm-level skinning and got nothing, then received the pet on their 3rd skin after switching to Legion Skinning.
Farm Antoran Hounds on Argus
Head to Antoran Waste and skin Antoran Hounds at 58.8, 15.4. The respawn rate is high enough to kill them in pairs as you loop the area. The pet dropped after roughly 547 fiendish leathers at this location, and reaching Fiendish Leather rank 3 during the farm may improve drop chances.
Alternative: Farm Duskcloak Panthara
Duskcloak Panthara in the Petrified Forest respawn extremely fast at 69, 71.6. Two Fossorial Bile Larvae dropped back to back during a six-hour session here, and one farmer went from Fiendish Leather rank 1 to rank 3 during the grind, suggesting this is a reliable fallback farming location.
Purchase from Lord Aj'qirai
If you prefer not to farm, Lord Aj'qirai in Uldum at 34.4, 18.4 sells the Fossorial Bile Larva directly. The pet is also available on the Auction House if other players have listed it.
