Hey guys, hopefully this isn't a repost. Anyway i found this randomly i went into my talents i put 5 points in Endurance Training, in Beast Mastery. I then thought, nah i don't want that so i went and re-learnt my talents, and when i did, my pets hp didn't go down! So i put another 5 points into the Endurance Training and it kept on increasing and increasing. So it's pretty simple, i'll explain it.
Step 1: Go into Beast Mastery Tab
Step 2: Put 5 Points into Endurance Training
Step 3: Go re-learn your talents and Repeat steps 1 & 2.
Now i'm not sure if this also stacks the 20% increase of damage for the pet, don't think it does. But anyway enjoy the glitch!
Oh one more thing, you can't teleport away from a custom teleporter or anything, or it will reset your pet's HP.