Yu is a very old Goa’uld who is the son of Ra with Hera. For a long time he was disinclined to chose either pantheon, but circumstances seem to have found him on Per Hathor, presumably having ended up choosing the Egyptians.
After having been thought to have been killed by Spec ops during one of their very early encounters with him, the sighting of ships bearing his symbol was somewhat of a surprise. It is widely expected that he has taken a new host, and therefore any previous images of him would be inaccurate.