Can monsters and monstrous calvery be knocked prone?

Sure, just read the relevant rule section for whatever is doing the knocking.

1) Monsters and Monsterous mounts cannot be knocked down by normal cavalry
2) Some things may knock down Monsters and Monstrous mounts. If it can/can't it generally says in that rules section.

Such as saying something like "cannot knock down anything of strength 6 or above"