No, it is a noun. A horse is a domesticated animal, an equine.
It can be either, depending on how it's being used. If it has a noun as its object, it is a preposition. The horse fell over. (adverb) The horse fell over the cliff. (preposition)
The preposition is up.
No the word every is not a preposition.
No, the word "when" is not a preposition. It is an adverb or a conjunction used to indicate time or a condition.
Yes, the word among is a preposition.
No, it is not a preposition. The word explore is a verb.
No, it is not a preposition. The word thing is a noun.
The word " I " is not a preposition, it is a pronoun.
No, it is not a preposition. The word away is an adverb.
No, the word "early" is an adverb, not a preposition. It is used to describe the timing of an action or event.
No, it is not a preposition. The word rivers is a plural noun.
No, it is not a preposition. The word has is a form of "to have" -- a verb or auxiliary verb.