In ancient Egypt the sacred bull Apis was seen as connected in life with Ptah, a creator god - but in death it was connected with Osiris, the first mummy, known as Osar-Apis
When Ptolemy came along, he took this old identification of Osar-Apis and gave it a new "flavor" ,so to speak, for his Hellenistic subjects. Serapis, as he was now called, was depicted iconographically as similar to Zeus, but he could also be connected in cultic power with Asklepius, Dionysus, and Pluto. Basically, if Isis was a goddess who comprehended most goddesses, Serapis was a god who comprehended most gods.