Abyssal Eel
Sold by Storm's Wake Supplies. When the Naga invaded Stormsong, they brought their favored pets: Abyssal Eels. Aquatic type. I found this item for 75 polished pet charms on in Millstone Hamlet, Stormsong Valley.
The Abyssal Eel is a pet vendor item available from multiple sources: purchase it with Polished Pet Charms from a stable master in Stormsong Valley, or unlock it through PvP pet battles with Olly the Pet Smuggler.
Guide awaiting review
Purchase from Stable Master
Travel to Millstone Hamlet in Stormsong Valley and enter the Inn to find the stable master. Purchase the Abyssal Eel for 
75 Polished Pet Charms.
Unlock from Pet Smuggler
Alternatively, win PvP pet battles until the quest 'A Shady Message' appears, which unlocks the Abyssal Eel as purchasable from Olly the Pet Smuggler at 45.5, 86.4. Horde players can also buy this pet from Olly in Dazar'alor.
