When people talk about prepatch/legacy pets, they are refering to three specific pet types (usually):
white wyrms
dragons
nightmares
how can you identify if any of these types are prepatch/legacy (untrained)?
White Wyrms will be 3 control slots and above 720 str. Legacy white wyrms did not spawn below 720ish str. When publish 97 came out, they change the spawn range of white wyrms. White wyrms now spawn with as low as 380ish str up to 760. Those with 380-510ish str spawn as 3 slot, those above 510ish str spawn as 4. You can no longer get a wyrm with 720ish str that is 3 control slots. The White wyrms should have between 433-500 hits no matter what control slot they spawn as.
Dragons that are three control slots are legacy. Dragons now spawn only as 4 control slots. They have just about 500 hits and 798-825 str. The only difference between the legacy dragons and the ones spawning now, are the control slots. Dragons have had issues since the original revert code was put in last summer. They are notorious for reverting and losing training points. When EJ comes out again they will most likely still be messed up (based on current testing).
Nightmares that are legacies are 2 control slots, around 300 health and somewhere around 500 str. The str varies depending upon the time frame in which the nightmare was tamed. Currently, Nightmares spawn in two ranges: ~250-300 str, which are 2-slots still, and ~300-520 str, which are now 3-slots. So the legacy nightmares have the str of a top end current spawning 3 slot.
There was little to no demand of legacys until very recently (past couple months). I was buying/given alot of them back in July/August/Sept and continued to get them until recently. Why there is a demand now, I am not sure... I'm guessing fomo. But most of the legacy pets I've seen trained have essentially been ruined. A lot of returning tamers are coming back to the game, and are trying to learn the new taming system by starting with their legacy pets, or simply uninformed tamers that don't know what they are doing and want a legacy for a status symbol. This unfortunately ruins most of them.