I've been trying to take this whole pet xp thing apart because I think it's been in since the beginning. I've dug up a few old threads on it that make for interesting reading, namely that people were asking these same questions in 2000.
I don't know for sure that pets taking 50% xp was in at launch but it was definitely a thing in 2000, so it's at least that old.
It also seems to have been that it wasn't something that was patched in, rather the ShowEQ guys started parsing xp and discovered the pet thing, and Verant was forced to respond. I'm still looking for these bits.
https://www.project1999.com/forums/s...d.php?t=338377