I am returning to UO a little late after all of you have done the long grueling hours of testing. Thank you.
I have trained about a dozen different pets, all on the Crazed Mage in Shame. I also found that tanking pets receive more experience. The amount per gain seems to be dependent on the its current lvl. A 1-2 slot will gain slow, a 3 will gain fast and will cap at about 50% and a 4 slot pet will train steady, but will cap at 25%, 50%, 75%... If your pet has 200-300 hp, you can probably have it tank. Be ready with bandies and G. Heals. The gains are really fast and training a pet from 1=>5 can be done in just a couple of days. I have done all my training immediately after taming, even with their skills in the 60s.
I hate it when the mage casts invis and most pets lose aggro. Also, I hate when the mage is low on HP and chains Greater Heals.
I tested something and found an incredibly EASY way to train a 1 slot pet as the tank even with only 125 hp. Have your character invis/hidden, but not your pet. When the mage spawns, let your pet auto-aggro. As soon as it starts to attack, cast Poison Field on the Mage. The mage will only use its spells and mana to dispel 3 of the fields then try to cure himself. He will dispel the tile to the left, then to the right, then the one he is standing in. (he is crazy) The mage will not cast any spells on your pet as long as he is poisoned. Time your fields about every 10 seconds and your pet will only take the 1-2 damage from wrestling melee.
I have done this successfully with a 1 slot Fire Beetle, Giant Beetle and Unicorn.