Ben
I think I can shed a bit of light.
Talking abt the possessive form "Its prey" is right, but when it comes to a name it is "Kevin's prey" or a specific person it is "the boy's locker room". I think an apostrophe is used when there is a missing letter or letters, so the way I was taught was to think of it as "Kevin (his) prey" with the apostrophe taking the place of the 'h' and the 'i' - the same with "the boy (his) locker room".It's same with the female only the apostrophe replaces the letters 'h,e &r' as in "Diana's dumplings" (Diana (hers) dumplings).
But then there is the scenario of "the boys" being plural when you say "the boys' locker room" so you can't use the apostrophe in the same place (and the apostrophe is replacing the letters 't,h,e,i,& r' from the word 'theirs'
so you have to put it on the end of the word - so imagine it as "the boys (theirs) locker room".
Still confused? Yeah and me mate.
good luck
Dave