I don't think chocolate itself would be outright poisonous to a horse, not in an emergency way. More like, just bad for the horse's health long-term. If you wanted to actually kill a horse with chocolate, you'd have your work cut out for you.

However, it does occur to me that chocolate would be an excellent way to conceal many types of poisons--namely, the ones with a bitter taste. Chocolate is already bitter, though it's sweetened with sugar. Mix a bitter poison into a chocolate bar, and you might have a poisoned candy that tastes like low-quality chocolate rather than like a murder weapon. Would it work in the real world? Who knows. But it makes for a plausible murder plot in a detective story.

1. Alright, a murder mystery based on...chocolate? Um. First, in canon, chocolate is more then perfectly safe for ponies. Pinkie eats copious amounts of it, and Derpy was shown drinking from a chocolate fountain in all of her wall-eyed glory. Chocolate cake has been consumed, chocolate chip cookies, ect. But, fine, let's ignore canon for a moment and see what chocolate does to horses.
   
   So, you know that chocolate is bad for dogs, right? Well, that is because of theobromine, a chemical found in chocolate, as well as some tea leaves and other foods. Fun fact, theobromine is derived from Greek, and translates into 'food of the gods.' Anyway, the reason theobromine is bad for dogs is because it can cause problems with the kidneys, heart, and central nervous system, building up over time until they cause tremendous problems. It is fatal to dogs, and can even be fatal to cats. So, what about horses? All of that was near useless information. Horses need a LOT of chocolate to affect them near at all. Due to ponies being smaller then horses, it would require less Theobromine to affect them then horses, but much more then dogs. However, the chemical stays longer in their system then humans, so it can build up to cause problems over time. How long? Years. Of insane amounts of chocolate.*
   
   However, Theobromine is banned form all horse races, as it is very similar to caffeine, and (supposedly) gives horses an advantage in races. Of course, it takes copious amounts to cause any physical effect on horses... Still, in acknowledgement of this, I would have horses get more energetic from eating chocolate. Like, if they had just a little too much caffeine. But, that's about all it does. As far as I know. Let's just say, most horse owners will recommend that you never feed them chocolate. It freaks them out. For some reason.*
   
   2. Most zebra body language is the similar to horse/pony tells. They flick their tales when annoyed too. Though, usually they flick their tales to get rid of bugs.
   
   *Don't take my word over some others. I just searched the internet and talked to my father who was (and in a way, still is) a horse nut. Should a veterinarian come in and say, "Yeah, what he said? That's all BS. Feed a pony a bar of chocolate and they will curl over dead the next day," then listen to the vet.