Tempted to say Mob would be a frog, but I remember Bikinireigen drew Mob as a turtle and that was also adorably fitting. Reigen as a snake seems a little too easy to pick but it works (his theme even sounds like a snake charmer's). He'd be a non-venomous snake, of course.

Also going to potentially out myself here and say I actually have penguin headcanons and think Mob would be an Emperor penguin and Reigen would be an Adelie penguin (they're known to protect baby Emperor penguins!).