Habitat
Rattleshakes can be found all over the multiverse, anywhere that magical food is present.
Many of them are integrated into monster society, though to what extent and in what way varies from world to world.
Horrortale
Horrortale and its AUs very seldomly see rattleshakes. There is little food or magic to space to such thing, and if by some miracle one is created in or stumbles upon the world, they are likely to be quickly picked off.