Welcome to the first installment of a semi-regular feature on the blog! I really like the concept of intelligent apes in roleplaying games (and if you do too then I advise you to buy a copy of Terra Primate asap ;) ). In this feature I will detail some Ape/Monkey-related fluff or lore.
Egypt apparently had several Baboon gods, among them the (NSFW) Babi/Baba, Hapi, and Astennu. Thoth also apparently got in on the act every once in a while as well.
So, let's look at the pieces we've got: grayish noble/ruler baboons; normal(ish) baboons; dog-headed apes.
A question before going forward is how anthropomorphized the baboons should; I lean toward "moderately', that is about 4 feet tall and quite capable of standing upright in a hunched manner. Real baboons are pretty violent but wouldn't work with a faux-Egypt setting unless they just took over the ruins, so I'd tone down the overt violence. Brutal formal duels/brawls would be an interesting way of conveying a strict hierarchical society with big anger issues.
As for magic, using Egyptian-inspired sun/star magic doesn't seem to particularly "feel" right with these horrible Egyptian baboons. Earth and water magic, focusing on stability, life, and fertility seem much more flavorful.
As for the "dog-headed" apes, they could just be an animal or a barely sapient race. Used as beasts of burden in peaceful times and war-beasts in war times (Bet you didn't see that coming!)
If an equivalent to Arabia exists and this Baboonian Egypt is as well-known as the real Egypt then there's really no way to make this revelation surprising unless the general character of the world is an insular/xenophobic one and the players are trailblazers.