WMG / Egyptian Mythology

Set's "typhonic beast" is supposed to be an "anti-animal"
It's very likely that Set's animal is actually a deliberately crafted mythological creature that is meant to convey unnaturality. Rather than a normal animal like a falcon or a crocodile, the typhonic beast is basically a mixture between a serval, a donkey, an aardvark and an elephant shrew with a demonic forked tail, something that never existed. What better way to demonstrate Set's attributes, Chaos and wild beasts, than to make up a fucked up creature that combines every weird mammal in Africa?
  • Throughout history, many unknown creatures are described as having the parts of more familiar creatures, such as the idea that platypi have "a beaver's tail and a duck's beak" or the ancient Chinese calling giraffes "Kirin". Set-animals could be a species that became extinct, from over-hunting (they would've been near some of the oldest non-migratory human civilizations) or when the Green Sahara turned into a desert and lost much of its biodiversity.

Ammit is the Grim Reaper of Egypt.
Its an Eldritch Abomination capable of devouring and destroying souls. While Osiris is the lord of the underworld he isn't a genuine embodiment of death. Ammit is The Grim Reaper itself, responsible for the "true" death that comes with devouring your heart and soul.
http://tvtropes.org/pmwiki/pmwiki.php/WMG/EgyptianMythology