Skeletal gorilla bones are dropped by the skeletal gorillas in the Ape Atoll Dungeon. Unlike regular bones, if these are left on the ground for too long, another skeletal gorilla will emerge from them and begin attacking any nearby players. If there are too many skeletal gorillas already spawned, they instead turn into "burning bones", which cannot be picked up. In addition, they cannot be turned into a greegree.
When players leave the dungeon with them in your inventory, a message will appear in the chatbox stating that "The skeleton bones you are carrying crumble to dust", and they turn into ashes. However, they can be safely smuggled out via dying and reclaiming the items from Death.
For an exhaustive list of all known sources for this item, see here