Can this sentence also mean "horses like (bla bla)" in a general way? Or would we use the plural like in English?
It'd have to be plural.
Sometimes we do accept both singular and plural in sentences like "A horse is an animal", but that's a universal truth. Not all horses like cookies. Some of them are just weird.
How would I say "The horse likes cookies" ? And not just any horse, but one in special... With 'To'?
Does ciasteczka mean strictly the kind of sugary baked goods that are usually implied with cookies, or would it also apply to something with a little more fiber?