Right-clicking a seahorse when close allows Naija to approach and climb on. Seahorses can only be ridden in Naija's normal form because they are controlled via song. A mounted seahorse swims significantly faster, and is especially useful for quickly crossing an area. However, they cannot be moved from map to map.
The Corrupted Seahorse can be located in Mithalas City but - unlike other corrupted sea creatures - this is just a cosmetic change. The Corrupted Seahorse causes no damage to Naija and can be interacted with like any other Seahorse.