I have been experimenting with this on the 2 slot mares, and from what I can tell there are some of these that when they spawn as the lower slot requirement they are VERY close to the threshold to bump up. Once you start training them, they gain a few stam/dex/intel/mana points and suddenly they are now strong enough to be bumped a slot. In addition to totally hosing the lower pet slot, it also causes issues between the follower slot on the character display and the follower slot on the animal lore display. In short, it's a complete mess!
@Kyronix @Bleak Can we please get the constant re-evaluation of the slot removed? Once a pet is tamed the ONLY way it should level is through the animal taming process.