Eternal Strider
Source: Pet Battle. It seems like a trick to the eyes, but the eternal strider can walk across water, its small feet never breaking the surface. Aquatic type. This pet was correctly added to the game in Patch 5.1.0. They spawn en masse throughout the zone at the edge of basically any body of water, regardless of size.
The Eternal Strider is a wild Aquatic battle pet found in Pandaria's Vale of Eternal Blossoms, catchable along the edges of any pool or stream in the zone.
Guide awaiting review
Travel to Vale of Eternal Blossoms
Head to the Vale of Eternal Blossoms in Pandaria. Eternal Striders spawn in large clusters along the edges of virtually any body of water in the zone, regardless of size.
Engage Eternal Strider Battles
Start battling Eternal Striders — they respawn unusually fast and appear in very large numbers, so you will never struggle to find a new battle. Both of their movesets include a non-damaging ability in the first round, which is worth keeping in mind when protecting a low-level pet.
Hunt for a Rare Quality
Rare-quality Eternal Striders do exist but require battling through a significant number of them before one appears, so keep re-engaging until you find one with Rare (blue) quality.
Tips
Flying pets or pets with Flying moves level exceptionally well here because most Eternal Strider battles consist of three Aquatic pets.
Eternal Strider battles occasionally include a Dancing Water Skimmer as a secondary pet, which is otherwise difficult to find due to phasing issues in the Vale.
Poison Spit's damage-over-time component can bypass the Mechanical passive revive, so avoid using Mechanical pets as your low-level carry here.
Leveling a new pet to around level 3-4 before using it as a carry is enough to survive the first incoming hit from an Eternal Strider in most cases.