According to wiki:

"Leo was one of the earliest recognized constellations, with archaeological evidence that the Mesopotamians had a similar constellation as early as 4000 BCE."

It seems that the Egyptians originally depicted this constellation as a lion, but later on due to the relation between it as rising along with the Sun at summer solstice, which relates to the heliacal rising of Sirius and the flooding of the Nile, then they decided to use a river animal instead of a lion. It had to look ferocious so they added another Nile animal - a crocodile on its back. The Senemut ceiling hippopotamus with the open mouth I believe depicts this constellation and Regulus in particular. During the old kingdom or earlier , Regulus being near the ecliptic represented the King in the form of a lion. There is a special relationship between the lion and Hellas, where naturally animals like hippos or crocodiles were not appropriate.