Learn Swedish: Vems pryl är det här? Jag kan inte identifiera någonting! - Whose item is this? I cannot identify anything!

Both work here, but någonting is an exclusively nominal pronoun, meaning it can only stand alone as a noun substitute (meaning "anything" or "something"). något is a dual-purpose form: it can act alone like någonting, but it can also be attached to a neuter noun like an adjective. Here, as a standalone object, någonting emphasizes the "thing" aspect perfectly.

Could I combine inte and någonting and just say Jag kan identifiera ingenting?

Grammatically, yes. However, Swedish heavily prefers the structure inte + någonting (or inte + något) over using ingenting as the object of a verb in everyday speech. Using ingenting directly as the object often sounds a bit stiff or overly dramatic to native ears.

Since pryl is an en-word (en pryl), shouldn't it be den här instead of det här?

This is the "dummy pronoun" rule in action. Swedish uses the neuter pronoun det when introducing, pointing to, or asking about the identity of something, regardless of the noun's actual gender. Since the speaker is asking "Whose item is this?" while pointing at an unidentified object, the abstract det här is required.

Is pryl just a direct translation of "item"?