I could be wrong, but I think it's coded so that in factions, a member can resurrect their war horse with zero vet. No other pets (that I know of) can be res'd by a factioner like that. I think it was intended to help factioners who otherwise couldn't get a pet res because of the "can't perform beneficial acts" block if they went to tram for a pet res, and maybe also to make war horses more attractive. There isn't a lower level of vet/lore required for low end pets, it's just the exception for war horses.
I'm going entirely by memory on the above, so you might want to check UO.com and see if there's mention of it in the playguide or somewhere.
Wenchy