Much easier to navigate google doc version:
+Christopher Weeks has very kindly automated the generator here: https://docs.google.com/spreadsheets/d/1Vtbib9wSVkbmEwsL3oaX8GKLASzXrHLCMPL0aHWt3Js/edit?usp=sharing
Roll a d4 and a d10 for each table (There is two: Form of the Beast and Aspect of the Beast).
Combine results for a fantastic beast.
Examples at bottom of post.
Form of the Beast:
Aspect of the Beast:
A roiling and bubbling suit of animated armour, ever melting and reforming.
Giant flying bugs that feed on magical energy. Becoming visible and active when spells are cast and burrowing themselves into the closest wizard.
Ghosts of long dead wyrms that form in the dying ashes and coals of a campfire. They will completely coalesce in the early hours and wreck havoc with a mocking grin.
All images are royalty free, gathered using the very, very useful: https://www.flickr.com/commons