In my personal Opinion I would say this is the most difficult of the big 4 to tame, they are somewhere between a Drake and a Dragon in Toughness. But for taming, very challenging :)
If you are not using UO3d (most people are not yet using UO3d, don't feel bad lol) You have to go into the bowls of the Terrathon keep in order to reach the mare spawn, it sounds tough and the first time for most people is usually even tougher (until they mark a rune to the second level of the keep)
The above diagram is for the Trammel Nightmare spawn, in felucca it spawns in the swamp between the keep and the mouth of the cave. Its much easier to tame mares in the felucca location, but getting there, the "dex factor" and frequent PKs makes it a toss up between Felucca and Trammel.
The things which makes taming mares so difficult is the other things around you in the mare spawn area, You have your dragons, Drakes, the occasional Terrathon that makes it back this far, and last but not least 2.1 million Ophedians.
Crowd control is the most important part of mare taming.
Spells like Paralyze, Paralyze field, Energy Field, Poison field, are good ways to get the spawn enough under control that you can lead a mare off and try and tame it without several other things to deal with.
Most of the players that make it this far back are generally pretty cool to each other, (although "Tame Jumping" is rampant, its tolerated a little more here). But things like Heals, Cures, and Rezing is a given. Other people will do it for you, and you need to always do it for them.
One of the reasons that there are usually not a lot of Jerks here is specifically because players have to rely on other players on things like rezing, curing and healing. Its a long walk to civilization and as a ghost, so lamers just don't get rezed, and that is usually the end of them :)
There are a few places that are marginally better than others in the general area. If you can get it away from the other monsters the taming process is similar to the main room in Destard, but make sure you keep yourself away from the other monsters, and remember that with the improved monster AI (artificial intelagence) they can see you through a wall, then walk around simple walls and get at you. If at all possible try and drag the mare out to one of these little side rooms so you can tame in a little more safety.
This room off to the side is a good one.
Even the Edges of the main room are better than the center
Be careful what room you "duck into" tho! :)
Sometimes the best (and only way) is to Drag the Mare all the way back out the entrance of the caves, through the swamp and into the keep itself where you can teleport into the pool and tame at your leisure.
There are a lot of other monsters that "come along for the ride" when you try and drag a mare that far.
Caution. As far as I know this is neither a bug or exploit, and its one heck of a feat to get an untamed mare all the way out this far, however there is No way for the monster to physicly attack you (even tho its spells can hit you) GMs in the past have blocked some safe spots, and might decide that taming from this location is a problem, unfortunately with OSI there is a sometimes a fine line between good tactics and what is considered an exploit. So use your best judgment on taming a mare from the pool.
If the dungeon is too Hot, and you don't want to risk the Pool, you might try the east swamp edges, there are generally only a few Drones that spawn there, not a lot of worry about, you can lead the nightmare there and set up your paralyze fields, and tame fairly safely.
Good luck on getting your Nightmare! :)
Side Note -At the moment this is the only Mare that is spawning, however Pure Mares (and Pure wyrms) are on the drawing board to make a return in one of the upcoming patches!
