Learn Spanish (Latin America): El caballo blanco corre como si fuera un unicornio encantado. - The white horse runs as if it were an enchanted unicorn.

The phrase como si (as if) always triggers the imperfect subjunctive when you are making a hypothetical comparison about the present. Even though the horse is currently running, it isn't actually a unicorn. English does something very similar: we say "as if it were", using a past tense form for a present hypothetical.

Is fuera coming from the verb ser (to be) or ir (to go) in this sentence?

It comes from ser. The verbs ser and ir share the exact same conjugations in the imperfect subjunctive (just as they do in the preterite tense). You have to rely on context to tell them apart. Here, "as if it were an enchanted unicorn" makes sense, while "as if it went..." does not.

I've seen fuese used instead of fuera. Can I say como si fuese un unicornio encantado?

Yes, absolutely. Fuera and fuese are perfectly interchangeable forms of the imperfect subjunctive. In Latin America, the -ra endings (like fuera) are much more common in everyday spoken Spanish, while the -se endings (like fuese) are generally reserved for literature or formal contexts.

Could the adjective encantado be placed before the noun, as in un encantado unicornio?