This is my first post yeah!
I was wondering if it was possible to increase a horse's strength, hit points, attacking, etc., by having it fight mongbats, zombies, or whatever?

You can train up their skills and "maybe" basic stats.

But make sure you bond with it first, before taking it out to fight stuff.

Welcome to the forums by the way

Welcome! And I have no idea.

You guys are fast! Thanks for the quick replies!!

It is because we have no lifes you see.

Hey! I resemble that remark!

UO isn't a life? Ohhh Noooooo....

All tameables can be traind to GM in any skills they start with (all have Wrestle, Anat, Tacs, and Resist. Some have Magery,Med,and eval as well and some have special skills like Rune Beetles have Poisoning and Cu Sidhes have Healing, etc.)

Also, all Stats (STR, DEX, INT) can be trained up to 125 if they started any lower than that.

Ive thought about training up my Blue and Fire Beetles so they dont instantly die all the time in the wild, but without Vet it gets kinda tedious. Not to mention that it take them a little while to kill Mongbats at first so its not really all that exciting...

yeah true, I trained my swampy up to high stats it took ages! and to be fair it can take a fair bit of damage, but doesn't really give it.. would have been nice if when fully trained or maybe when it reaches a high level of training it would do what it does for chaos lord riders and breathe fire.........

By the way...when your horse or swamp dragon has indeed gained a little increase in experience, will the Journal Log let you know about it? You know, that little script in the bottom left corner that says stuff like, "Your Focus has increased by 1." If it said something like this about my horse, i must have missed it. Unless of course it truly does take ages.

The only real way i can think of is with animal lore, i could be wrong.. but i have never seen any gains in the journal or on the screen when any pets gain in stats/skills.... the way i did it was get my tamer to train him and keep an eye on his skills then trade to my warrior

The only way to tell the gains for animals is by using animal lore. What shard are you playing on? Maybe you can hook up with someone from this forum to lore your horse to see how it's doing.
Archaon...I have trained up my blue beetle some, unfortunetly he thinks he can kill anything now and takes off on a whim. My fire beetle is still a wimp. He takes fooooooorever to kill a rat or mongbat.