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?
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.