I agree with a previous poster farming them a bit and you can do better.
I take a Mare, Cu or Dread in to do the Killing. I invis, wait for it to spawn, lore it to decide if it is an upgrade or not. If it is not then I kill it.
I am very confident the mentioned HP's and strength are low, not by a lot but I am confident they are low.
I am fairly confident I have seen a mare at 325 hp (just spawned and no buffs) and a strength of 540. I did not tame it as it's resists were crap, compared to stratics information. I believe more in Resists than I do in Stats.
Just my 2cp's on the value of Nightmares
A Digression, Nightmare's were a superior pet when I left, 8 months after Trammel went live. One could take down an Ogre Lord with very little need for me to heal or vet it.
I come back 13 months ago, work my rear end off to get to the ability to tame a Nightmare. I take it to the Ogre Lords (Wanted the gold, fame and training). It required constant veting and healing. I parked it in the stables and moved on.
About 8 months later and a good deal better informed on the changes to Pets and Taming, I get the Mare out and take a long hard look at it (I am crunched on stable space). I have a pair of Dreads on each account, a Pair of GD's on each account and a Pair of Cu's on each account.
I get information from here suggesting that some time between when I left a returned that mares, wymrs and dragons were nerf'd (cut in half?).
I look at that Mare really hard. It's wrestling, tactics and anatomy are all at 100 and the Ogre Lords still chew it up like it was nothing. Not as bad as when I first tamed it but yeah it was a wimp compared to what they were when I left.
THEN I SAW SOMETHING, something I clearly did not see before. Its magery and evalint were in the 20's or lower. The Nightmares Magic spells were always hard hitting. I could not bring myself to believe this mare with a 20's Magery and Evalint were doing any significant damage.
So I decided to spend the time sink and get them both to 100. Now that Mare is about equal to my memory of the Mares before I left.
Would I take a Dread Warhorse in vs a fully trained Mare and a Fully trained Bake Kitsune? No thanks, I will take the Mare and BK.
Would I take a Dread Warhorse (yes fully trained) in vs a fully trained Cu (yes 100 healing)? Probably not, but this is kind of iffy. It is a clear answer if I may not be able to stick around to vet, then it is the Cu. The rest depends on the matchup of the mobs attack vs the pets resists and the pets attack vs the mobs resists. The two (Dread and Cu) are that close.
To say a Mare is worthless? Not in my book, for a 100 (GM) tamer, the Mare and Bake is a darned good combo for the investment in skill points.
Unlike the OP, I would get my taming at or over the To Tame requirement for the Cu. That is just my opinion.