A creature threatens the squares in which it can make a melee attack, but does that include its own space? (If it's relevant, assume it's a medium creature.)
[RPG] Is a creature’s square included in its threatened area
opportunity-attackpathfinder-1e
Best Answer
Yes, a creature threatens squares it occupies
As Hey I Can Chan stated, the section on Attacks of Opportunity says:
However, a section from Big and Little Creatures In Combat helps shine a little more light on the situation...
(Emphasis added, note the tense difference between "they" (Tiny/Diminutive/Fine creatures) and "you" (player, assumed Medium by the Player's Handbook unless stated otherwise))
This shows that a creature is capable of attacking into their own space, which combined with the Threatened Squares text saying you threaten all squares you can melee attack into, indicates that yes, you threaten squares you occupy. This is further supported by natural reach for larger creatures including the entire area up to their maximum reach as threatened.
The use of the line "Generally, that means everything in all squares adjacent to your space" in the first section is simply meant to indicate the situation that will happen a vast majority of the time, not to indicate a restriction, as presumably most combats will not involve two creatures sharing the same space.
Indirectly related (but currently a hot topic in the comments), these are the rules regarding Tiny and smaller creatures ending their move in another creature's space: