this rule apply only to humans?
No, the massive damage threshold applies to all creatures except
constructs and undead (note: unlike other creatures constructs and undead also die and are irreperably destroyed when they reach 0 HP instead of -10 HP). Outsiders, including Demons, are
subject to massive damage (and critical hits and sneak attack) though it is a common mistake to think otherwise. Certain specific creatures may, of course, be exempt but will say so in their entries.
It is a common house rule
to increase or decrease the massive damage threshold of creatures other than medium size by +/- 10 points for every size different than medium. This does make the game harder on the players.
Hope that helps.