Like you, I have two old mares and one meat-eating unicorn. I traded my fire-damage frost spider to a regular poster here and I really, really hope he'll post the lore pages of that lil critter just because it's fun!
My carnivorous unicorn has within range stats and shows ---- for physical resist. Because she was gifted to me, I don't have much specific info on her.
My pre-patch shortmane mare converted normally to 515/315 which was, I believe, the default max setting for pre-patch nightmares. Call shows resists well within the mare resist range.
My longmane blue mare converted with 524/315 with the normal within-range spread of stats & resists.
The only difference between the two mares is that the account with the shortmane was open at the time of the conversion while the account with the longmane blue was closed and reopened some years/publishes/patches later.
I do have another pre-patch longmane blue (always preferred those to the pure black mares...) on a closed account which I checked during the last, recent Return to Britannia week and which converted normally to 515/315 & within-range resists.
Screenies to follow at my earliest opportunity.