For me when i choose a pet (one i will keep)
First, i check resists,
-Any that has not phy resist max or very close to max is a pet i dont even consider, no matter the rest. Reason being almost all monsters do at least some phy damage and many do only phy damage.
-Then, i want the strong resist of the pet to be close or very close to max. Simply because it will be my "pet of choice" when dealing with monsters that hit hard in this element. (the difference between a 80% resist and a 90% resist is double damage).
-Then, i want all other resists good (no more than 5 from the max), the less important being the weak resist of the pet. Reason being i wont use that pet in a place where most monsters hit in the weak resist.
-If, and only if the resists are good i will check the other stats.
-Any that is not close to max life is discarded.
-The ones with high dex seem to run faster, so i like they have high dex.
-Strengh and intel are not that important for me (2 or 3 damages difference between a max and a min stat). Sure, if those stats are good too, its better
All that means that a keeper is not an easy pet to find and you will probably have to kill many before finding the good one...
Now, obviously, all that is not super important for a mount because it will not be a tanking pet, but i still cant play with a "bad pet" and im always searching for a better one
For the equations you are asking about, mmm, im not sure i understand what you mean.
Your unicorn with 210 life and 55 phy resist being hit with 100 phy damage will take 45 damage, so 5 hits, she is dead (but probably you will heal her and sure she will heal herself so...). A 65 resist phy would endure 6 hits at 35 each allowing you easier and less stressfull heals (both from yourself and herself).
I hope it what you was asking for...