I cannot explain but I have noticed the verb "gustar" is conjugated differently than most the verbs I have seen. http://www.123teachme.com/spanish_verb_conjugation/gustar Using that chart I assume if it was one horse you would say "¿Te gusta el caballo?"
It functions the same as all other Spanish verbs but does not translate directly into "X likes Y" because they don't have such an expression in Spanish. Gustar means to please and it's what they use instead. Te gustan los caballos? Do horses please you/Do you like them? A ella le gusta fumar, To smoke pleases her/She likes to smoke. Te gusto? Do I please you/Do you like me?
Because "you" is the object and the recipient of the action (gustar, received from the horses). The full expanded sentence would be "A ti te gustan los caballos?"
On the other hand, to reverse the question and ask if horses like the person, you'd say "(Tú) Les gustas (a) los caballos?" or "Los caballos les gustas?"
I actually changed my answer (which would have been counted as correct) because as I thought about it more, it seemed that the gustan referred to what the horses liked, not what you liked. So I answered "Do the horses like you" rather than "do you like the horses". Why does the verb agree with the horses rather than the person being asked?
why is it "a les gustan los caballos" but "te gustan los caballos" (no "a")? two questions ago I was marked wrong for not including the "a", so OK, I need the a for the "to them, the horses are pleasing" (They like the horses). Why is this phrase different? Are both correct? Is it because one is a statement and the other a question?
Also, it doesn't matter if it is a statement or a question---
"Les gustan los caballos." = "They like horses."
"¿Les gustan los caballos?" = "Do they like horses?"
"A ellos les gustan los caballos." = "They like horses."
"¿A ellos les gustan los caballos?" = "Do they like horses?"