What constitutes horse abuse?

There is an Appaloosa stallion that I drive by frequently. The people don't have any other horses, but do have maybe 10 head of cattle that graze out in one of their fields. The horse however is always in a tiny bare pen. I have never driven by when he wasn't just standing there being bored. The pen is an odd shape, but somewhere around 25 or 30 feet long and 20 feet at the greatest width.
It just doesn't seem like a healthy amount of room for him since he never gets out.
Could this be considered abuse? I feel terrible for this poor horse everytime I drive by. Is there anything I can do?

You'll have to check your county's neglect statutes. Odds are that if he has food and water there is nothing they can do. A horse *may* need some form of shelter, depending on where you live, but I wouldn't count on it.

Keep in mind that the horse may get turnout or stall time that you just haven't seen yet, too.

The people that I bought one of my horses from had 3 horses in pens made out of four panels (one on each side to make a square), no shelter. They did truly love their horses though, and I think it was temporary until their barn was finished but I remember it was late May (80+ degree weather) and felt Horrible for them. Where I live if you have hay and water, no matter how often they feed the horse, it isn't considered abuse until the horses is very noticeably skinny, and I've never heard of anything done about improper shelter.

Maybe he has a physical problem that requires limited turnout/activity. There is so much more bad stuff that can happen to a horse besides being bored. If he looks healthy I wouldn't worry about it though he would probably be happier as a gelding with friends.

As the others have stated, you don't know what happens with this horse the other 99.99% of the time when you're not driving by, nor do you know whether or not he has some sort of disability that limits his turn out.

Just because he's standing in a bare lot doesn't mean he's being neglected or abused. If he has water and appears to be in good health and a decent weight, he's a lot better off than many horses nowadays.

There was someone who would drive by the barn I board at everyday right after we would do turn in, so they never saw the horses outside. So they assumed the horses were never outside and decided to report us to the MSPCA who then came out and found there was no problem, obviously. The problem with driving by is that you only see a snippet of their day, you really have no idea what the situation is, I agree with the others that unless he looks unhealthy he's probably fine.

If the horse is indeed a stallion, the pen size may be regulated by state statutes.

Echo what others have said. Unless you are there 24/7 you have no idea what happens with this horse when you are not there. There is frequently a very good reason for a horse to be in a dry lot.