What is hatched egg for?

[Splif°]

It's an active item which i cannot use? Is it used in a quest?

[Jackalope_908]

Hang onto it until you gather enough rads to mutate. If you're holding it when that happens, you SHOULD get the Death Roe exomutation, which involves laying eggs from a newly formed cloaca that hatch into little spider-like mutants with your head on them.

There's other items that do the same thing, but so far I've only seen the boomerang arm and the hat from the Home Slice mutation.