Many of the great apes have a sagittal crest, which is a ridge running along the skull that allows for stronger attachment of chewing muscles. Male gorillas have particularly large crests, which may be a form of displaying mating fitness in addition to its practical use. It is plausible that in some circumstances, runaway sexual selection may promote larger and more elaborate crests, particularly in males.
However, it is unlikely that simian horns would ever be used for fighting. In ungulates, head-butting is a fairly effective means of attack even without horns, thanks to their high running speed and low positioning of the head while charging. This promotes the evolution of hard plates and sharp protuberances that increase the effectiveness of the head as a weapon. Simians, by contrast, tend to bite, scratch, and strike with their hands while fighting. While a primate in a pinch may employ its head as a weapon, it is unlikely that this would ever become their go-to means of attack.
So if a simian species developed horn-like growths on its head, they would probably be more akin to the crests of birds than the horns of ungulates: sexual displays, not practical weapons. Light weight would be more important than strength. I would expect the bony sagittal crest to remain, but the structures on top would be more lightweight and probably shaped in such a way that would minimize the balance problems they would cause in the trees - wide and flat, rather than tall. These crests would probably be more prominent in more robust apes that tend to live on the ground and fight or scare off predators rather than run away. It is also possible that they would be covered in brightly colored fur to increase their visibility.