Good to see you here, Mo!
I'll take a stab at this and I'm hoping more knowledgeable minds will chime in to correct me wherever I'm inaccurate.
Before pub 16, tamers weren't able to see very much information when using animal lore skill. While I was certainly there at the time, it's that long ago that I don't rightly recall exactly what info we did have available to us in the animal lore pages, though I remember we did not have skills displayed at that time.
Basic & imprecise to be sure, however, simply by hunting/killing with a mare you had a "feel" for whether a mare or other pet was actually any "good."
With publish 16 came more info (eg, we saw skills & skill levels of our pets for the first time ever) and it was then that (amongst other things like no more orneriness with increased owners) there was a standardisation of stats/resists so that both fell into a set range.
Implementing that set range of stat/skills/resists was...well, let's say, `imperfect.'
Pre-patch mares *should* have been given a blanket of 315 (the max) hit points, and 515 strength.
While
most mares were successfully converted, there were some unintentional & apparently purely random variances, both to the stats as well as the resists. I have not myself seen a mare exceed the GM skill level cap.
I have a pre-patch mare which exceeds the set pre-patch range in it's strength (should be 315/515 and is 315/524), though its resists fall well within the random set range of resists. Others have mares with no displayed resist in, say, physical resist where it displays simply as "-----" instead of a number.
Fresh spawning post-patch mares benefitted from a rather wider range of stats, specifically: a max of 298-315 hit points and 496-525 strength and resists as displayed in the Hunter's Guide re:
Nightmare .
Since a pre-patch mare is not visually distinct from a mare I tamed 5 minutes ago (not in 2d and certainly not in KR where they all appear identical), there isn't, to my knowledge, any huge tamer or tamer-collector interest in them, unless, perhaps, they show ---- in a resist or have abnormally high or low stats/resists.
From a purely non-scientific standpoint, and pixel-softie that I am, I wouldn't part with my own pre-patch mares simply because I have fond memories of all our adventures together.