Suggestions for a name for the venom? Like Demiurge, my first instinct was chaos poison...then I noticed they were neutral evil. Not that being neutral precludes any use of "chaos". Maybe "gloom venom"?
I was a bit concerned at first with 3 HD for a Large creature, although a Large viper does indeed have 3 HD. And some other Large magical mounts have similar HD (hippogriff=3 HD, pegasus=4 HD).
So let's stick with 3 HD, but go ahead and give them the DR 5/cold iron. Even a CR 1 Tiny grig has DR 5/cold iron.