Ultimecia's Griever

[Evol]

Selphie mentioned that she found a GF inside a monster, what could that mean?

and Siren can be drawn from Elvoret.

I think that GF fought with that monster and almost defeated.
So that GF chose to junctioned itself to that monster instead got killed by that monster.
That's why Squall can draw GF from Elvoret.
Well that's just my personal opinion.

OR monsters can learn Junction ability themselves just like Squall and co.