In modern English, the past participle of come is come. (In the 14th and 15th centuries, it was sometimes comen, cf. driven for the verb drive.)
This come is that past participle of come. This is a somewhat archaic, or regional and vernacular usage of this particular participle, but it is not uncommon to use the past (or present) participle of English verbs as adjectives in the postnominal position. Consider:
- He's a creature summoned straight out of a nightmare.
- It was a car driven off the road.
- They were an army drowned in the tide.
Whenever a past participle is used as an adjective in the postnominal position, it is of course possible to add a helper verb and relative pronoun as its subject:
- It was a car (that was) driven off the road.
- They were an army (that was) drowned in the tide.
However, the helper verb and relative pronoun are not omitted from the original sentences, and neither are they omitted from your example:
- He's a creature (
who has) come out of a nightmare