Easy answer:
A Vollem is a Nightmare that can't be ridden, that requires no taming (and of course got the spiffy "Kitty-goyle" art that came about because of all those silly threads with CatHat, back in the day).
Look at the Nightmare's page in the Hunter's compendium on the UO Stratics main page.
The constructed and crystal/bonded ones are identical, other than the contstructs perma-die and can't be bonded, while the crystal-source ones insta-bond, and (unlike other crystal summons) can be transferred between owners (requiring rebond time).
Even the loot you find on a Vollem from a crystal is identical to what comes off a Nightmare, on its first death.