Let's say someone says "It started." out of nowhere. "It" doesn't have an antecedent. How would you qualify such a pronoun? I am thinking "dangling pronoun", but the term is rarely ever used, so I am thinking there might be a more formal word or term for it.
it without an antecedent is a dummy pronoun: https://en.m.wikipedia.org/wiki/Dummy_pronoun
However, in “It started”, as Jason Bassford points out, we don’t know what it refers to. This is probably not a dummy pronoun. Something started. This it is different from the it in (for example) “it rained”, where it definitely has no antecedent.