The only difference i know for sure is that a "pre-patch" WW will not eat gold. Whereas all WW's tamed after the "patch" eat gold or meat to sate their hunger
In general i think that if a pet is "pre-patch" it is usually weaker than its current counterpart. For example, some "pre-patch" nightmares have 0 physical resist (its displayed on the gump as --). This might be something you’ll want to look for when identifying if the pet is "pre-patch". There may be exceptions to this (perhaps some pre-patch pets are stronger in one way or another), but I don't know of them specifically.
If you’re looking for a fighter, get a current incarnation of the pet because it will generally be less expensive and stronger (or just as strong) than a “pre-patch”. If you’re looking to spend tons of money on a rare pet, then "pre-patch" is right up that alley.
Perhaps Petra, Lynk, or another pro can turn my post into a flaming ball of lie, but this is my understanding of the “pre-patch” situation.