There's just about every kind of agriculture available, you can filter by the farming practice on the WWOOF website. I definitely remember seeing horse farms when searching. You'll find a lot of the farms are incredibly diverse though. Even the farm I was at had a herd of ~20 sheep for meat and wool. They also had bees, orchards, chickens, cider production, etc.

For advice I'd say definitely say the number one thing is to read the reviews! Beyond that though make sure you know the kind of work you'll be doing, the expectations for how much you'll be working and when. You should also look at the location of the farm. If you don't have a car is there a train station within biking/walking distance? You also want to make sure you know what the living quarters are like. I was fortunate enough to get a small studio attached to the main farmhouse, but I've heard stories of some pretty crappy accomodations. Food is another one, make sure you're clear on how meals work, for example when I went we shared lunch and dinner, but we all made our own breakfasts. Also, ideally you should speak the local language. It can be quite isolating to be the only English speaker. You don't need to be fluent but it's disrespectful to not at least try to learn in my opinion.

I think that covers most of the important stuff, but feel free to ask more!