Nightshades are from the OD&D (probably introduced in the Master box set) by way of the AD&D Mystara MC Appendix. Dire animals are mostly 3e-speak for 2e's Giant animals (e.g. Rat, giant).
Avolakia are dynamic in their uses since they polymorph themselves easily. They can be a powerful and unpleasant surprise for a party that thought they were just dealing with an evil cleric, but then find that he is really this worm-thing.