Poseidus has a 100% chance to drop Reins of Poseidus, which is a bind on use seahorse mount that can be summoned anywhere in game under water. I have seen very few of these on the Auction House on my server. The few I have seen have generally be listed for about 100,000 gold. He also drops a hefty amount of gold, when I killed him he dropped around 99gold. He also tends to be very generous with other loot, often dropping item level 359 bind on equip gear and pricey formulas and designs. (When I killed him he dropped Blockade's Lost Shield and Formula: Enchant Bracer - Major Strength.)
|Subdued Seahorse - Learned from Reins of Poseidus|
I will be honest here, I was a little lucky in finding him. I have not really actively camped for him like I did for other rares that drop mounts such as the Time-Lost proto Drake and Aeonaxx. Every morning when I'd log in before doing dailies I would head to Vashj'ir and swim over each spawn point just to see if he happened to be up. I was awfully surprised when my npcscan went off for him this morning. (This is actually the second time I've found and killed him, but the first was toward the beginning of the Cataclysm expansion before he dropped any loot.)
As you can see in the video below I found him in the Underlight Canyon spawn point. He pretty much roams around in one spot so you don't have to look far at all from the points highlighted on the map. He was also not swimming too high off of the sea floor. The first time I found him was just above the Ruins of Theserai Temple, also not swimming too high up. Basically high enough in the water to be out of range of the hostile naga down below.
I have read that flying above the water is a viable way to search for him as NPCScan will pick him up. Although this way may be faster to get from one spawn point to the next, if he is found making your way down to him will take valuable seconds where someone else could snag him. If there is a lot of competition in the area and you plan on AFK camping this guy with NPCScan I recommend sitting your toon right on a spawn point under the water on your Abyssal Seahorse. If you don't already have the Abyssal seahorse, do the first few quest lines in Vashj'ir. It is given out as a quest reward shortly after you start questing there.
Once found, the actual fight with him isn't difficult. He has a couple of attacks that involve bubbles and he'll attempt to run away and heal. He's basically a paladin with fins.
He is also one of eight rare spawns in Vashj'ir. All of them are:
- Ghostcrawler (Spirit Beast Crab) - For lvl 85 BM hunters, can not be killed
- Mobus <The Crushing Tide> - Group Boss, drops a nice epic polearm
- Poseidus (Sea Horse) - Drops Reins of Poseidus, a bind on use underwater mount
- Shok'sharak - Faceless one, drops some booties
- Lady La-La - Drops a nice level 80 healer trinket