Abyssal Seahorse mount is trained from Subdued Abyssal Seahorse, which is a quest reward from  The Abyssal Ride.
The tooltip for this item indicates that the mount will work in any aquatic environment. However, the tooltip for the mount itself says that the Abyssal Seahorse will only work in Vashj'ir.
- [Reins of Poseidus] appears to do the same thing, but functions outside Vashj'ir.
- Abyssal Seahorse travels at 450% of normal swim speed, regardless of what skill level Riding is at.
Patch 4.0.3 (15-Nov-2010): Added.