Melliandra, I'm wondering if the following might be what happened with your pack horse that bolted as soon as you dumped some logs into its backpack. (And by the way, in case you missed this change while you were gone from UO, you can now chop logs with your axe and they'll turn into boards. Don't need carpentry anymore to do that with the better quality wood either. If your lumberjack can chop the logs, he/she can also cut those logs into boards!! No more carrying saws and no more needing to have carpentry to cut those heavy logs into boards.)
Okay, back to this hypothetical scenario:
1. You logged off earlier in the day while your faithful pack horse still had something in his pack. Your character went off to La La land and does whatever happy lil characters do in La La land, while your pack horse was left alone, cold, scared, hungry, and still loaded down with a heavy weight in your house or inn or wherever else you "abandoned" him. He didn't automatically get sent to the stablemaster to care for while you were gone because you forgot to clean out his backpack!
2. Sometime later, you logged back in and saw your trusty old pack horse still standing there and decided, "Let's go chop down some trees, old and faithful friend!" Unbeknownst to you, old and faithful friend was mighty upset with you for leaving him alone all that time with some of your possessions to worry about and with straps still chafing his sides and belly. His happiness had slid way down the critter Loyalty scale, maybe past the neutral zone and into that area where pets are contemplating, "Should I kick the snot out of her when she comes back or just take off?!" (Go here if you would like a rather more technical explanation of the critter Loyalty scale:
Professions - The Tamer: Animal Lore.) Did you reach into your pocket, pull out an apple, polish it up a bit and offer it to old and faithful friend as a thank you for his fine service of late? No, you did not!
3. You recall off to your favorite wooded area and old and faithful friend is sent along with you by strong forces of magic and a still rather tenuous bond that exists between you and him. His natural instincts still haven't gotten the better of him yet and at least for a few seconds he submits to your desires.
4. Pulling out your axe, you come to a fine looking specimen of a tree and proceed to hack away at it until you're completely overloaded with beautious logs. Turning to old and faithful friend, you unceremoniously dump the whole pile of them into his pack, and clucking your tongue, you say, "Follow me old and faithful friend! We have more trees to chop down!"
5. At this point old and faithful friend has JUST HAD IT. No apples. No nice long drink of cool water at the stable. No thank you's. Just more damn stuff to carry! He's outta there, not even gonna give you a parting kick.
6. And so, old and faithful friend shall nevermore be known as YOUR old and faithful friend. He's decided to take his chance at freedom, even if it means he will forevermore roam the land carrying those packs you once tied onto his back. Yes, eventually he'll drop that last load of logs and whatever else you left him with that last time you went to La La Land. But he's had it with two-legged creatures. Good riddance! If anyone in the future wants his company, they will have to tame him and treat him exceptionally well to overcome his confusion about two-legged creatures.
The End.
(Anyway, the moral of the story is don't log out and leave your pack animals behind with stuff in their pack and FEED THEM once in a while to show your appreciation! Feeding them usually restores them to a Wonderfully Happy state. Bossing them around when they're in a neutral or negative state may just cause them to slide further down the Loyalty scale until they give up on you and go wild. You can retame them yourself, even without taming skill, if you can catch them. But once you have them retamed, you must feed them to get them out of the "Confused" state, give them a command, and then wait a week for them to form a new bond with you.)