As far as my real life experience goes, I'd have to say the theories are valid. I should also say as a disclaimer that my filly, Nova, is the first horse I'm training myself from start to finish. Regardless, the psychology is sound, regardless of species. It's easy to use behaviorism with kids and dogs especially.
There are two key factors that come into play more than any others with behavioral theory - consistency and timing.
Consistency is pretty self explanatory - It may be cute when a newborn foal nibbles your fingers, but it's not so cute when those big teeth come in, so don't permit nibbling in the first place and you'll never have to fix the problem.
Timing, in this context, means that rewards/punishments need to be given immediately, in relation to the behavior. To use the nibbling example, as soon as the horse makes to nibble, tap his muzzle to get him/her to back off. If you wait until they've been chewing your shirt for 5 minutes and then tap because you're annoyed, then the message isn't as clear.
Timing rewards is equally as important, even more so when the horse is learning something new. As soon as the horse takes the desired action praise them right away, don't save it up for the end of the training session or they'll think they're being praised for walking nicely back to the stable.
I would say the third most important factor is motivation, meaning what motivates your horse. With Nova, I'm still learning her preferences, so I'll use my dogs as an example:
Jack is completely and utterly driven by food. He will do anything for a cookie.
Sammie, on the other hand, wants her toy more than anything in the world.
If I want to teach Jack a new trick, I'll use food rewards because that's what motivates him the most. If Sammie's been misbehaving, I'll take away the toy for a while.
I guess it would be like a horse that will not cross water under any circumstances... kick, whip, push, pull, yell... until someone's standing on the other side with a peppermint. If that's the case, then you know what reward motivates that horse.
Eventually, you get into the realm of conditioned responses (Pavlov's dogs). The theory is a little different, but there's a lot of overlap. For those not familiar with Pavlov, here's how it worked:
Pavlov rang a bell, fed the dogs a bit of meat, then measured their saliva production.
After a while, Pavlov could ring the bell and the dogs would slobber in anticipation of the meat to come afterward, regardless of if they actually got anything to eat or not.
The dogs were conditioned to expect meat at the sound of the bell, even though the bell itself originally had no association with food at all.
Horses exhibit conditioned responses as well because it becomes a part of their routine.
I'll let you all give the examples for this :)