They are immune to runspeed changes, which is inconsequential, as they don't run at low health.
As a 55th level Paladin, I didn't notice their MR to be unusually high, as probably 80% of my stuns hit. With Glamour of Tunare on them, virtually all the stuns hit.
Pacify works nicely on them for single pulls.
Try to fight them with your back to a wall, as you will get knocked around quite a bit. Helps to keep your spell interrupts to a minimum. They stun frequently if you have your back turned, which is normally the case on a long pull.
This mob has can be the PH for other seahorses including Fellspine. A few are roamers on a certain path and several are from static spawn points. They can NOT be snared; can be slowed; can be kited underwater if your swim skill is 200; does not summon; will not leave the water if you go up top (but watch out for other mobs); is social and will not run if two or more are fighting (including seahorses), will pass aggro if they get away, and attempts to run at low health if it is alone. They drop in addition they drop: words/runes, Ancient Fishing Pole, and all the chipped-type gems.
These see thru invis - just in case you want to run through to WW on your own like I did. Oh, and they hit hard and fast - at approx 1000AC 2 whipped about 2k HP off me in ONE round. Be careful!