They have better resists and their HP range from nightmare level to much higher. They can have higher dex and have higher int than mares. Their rage attack keeps them after stuff and attacking, even if the target invises.
In a one on one fight, with both creatures at same skill level, the kitsune will win. However, you can't ride them. They are not at the same skill level as any of the 3 slot creatures. The kitsune being a bit more powerful is balanced against the mare by the fact you can't ride it.
Think about it though. Nightmares were truely just that when they came out in '98 with The 2nd Age. When AOS came out, the slow gains in player power that made creatures tough and gold hard to get suddenly changed. The limit on player power was removed and legacy creatures where capped at a rather limited level with bad resists. All the new creature types that came out after AOS have progressively been a bit more powerful and had better resists than the legacy creature types to try and make them more in line with player power. The Kitsune being many years removed from when nightmares came out, have resists more in line with what creatures should have versus what players do have. For the nightmare to still be used by almost everyone proves it's power as a ridable mount.
I love kitsune. I have three ok ones in my stable looking for homes (survivors of my search for another good fox) and my two really nice ones that I use. I'd never give up my old mare though. I've had her since well before AOS came out and before we could lore everything on pets. She's an old friend who I still take out when I need to be able to tame 3 slot creatures and ride. I use her quite a bit still with a rune beetle on a ball to help if I need to kill something that is to powerful for a two slot pet to kill. I run a stealth/tamer a lot so even with that char on foot anyway most of the time, I still use my mare.
I think both kitsune and mares should be in everyones stable. They both have uses in different situations.
