Ye Olde Inn

Can the chaos spells be used at anytime by monsters with mind points?
or is it just certain monsters listed in the quest book that can use them?

Nevermind I found the info I was looking for in the manual.

Well, I had already mostly worked up this answer, so for others' benefit...

Your answer is noted on p.21 of the North American Instruction Booklet under Zargon, the Evil Sorcerer's Turn:

Note: Only certain monsters may cast a
spell. Refer to the Quest Notes in the
Quest Book for more information.

It is reinforced again on the next page under Action 2-Cast A Chaos Spell:

As Zargon, you may cast a Chaos Spell
instead of attacking. You must give your
Chaos Spells to specific monsters as
called for in the Quest notes....

The rulebook doesn't have to necessarily limit your options, however. The very basic nature of the game invites house-ruling left and right- in this case by modifying a published Quest or by creating your own. As long as you call a monster out in the Notes section of a Quest, just about any monster can cast a spell or two. As your question indicates, a Skeleton, Zombie, or Mummy normally wouldn't make a good spell caster, as they have zero Mind Points. A Goblin, Orc, or Fimir could logically cast a few spells as a named monster. For a twist, a Chaos Warrior or Gargoyle could be a spell caster in a Quest. Since the monster figure doesn't actually have to be limited by the base monster's stats in the named monster's description, even a Mummy with Mind Points could be created to cast spells.