Apparently Monthu is derived from Ra, the Sun diety, and embodies the Sun’s schorching power, eventually becoming a diety of war. So their similarity makes sense. Both of them carry the Sun disk on their heads, and Monthu also carries two feathers. Khonsu on the other hand carries the Moon. And Horus wears the combined crown of Upper and Lower Egypt, as he is the pharaoh’s guardian; the symbol of the falcon could represent Horus or the pharaoh, though as far as I could figure out that meaning isn’t invoked by the form of the other three dieties.