But the same is true for non-standard monster abilities. Make it too complex and you will need a lot of space to explain it.
Agreed. Which leaves you with a couple of choices:
-Don't use complex abilities on any monsters ever
-Use complex abilities on monsters but explain it in full detail on the monster and allow that much space in the stat block to give that information
-Use complex abilities, summarize them on the monster and refer to a spell list or other reference to explain the details.
For the spells you mention, most monsters might only have one and that in a limited manner. For example,
teleport (to lair only, no failure chance; PHB XXX), or
polymorph (to human only, statistics below; PHB YYY).[/QUOTE]
I'd be perfectly ok with that. But it doesn't have to be based off a spell list to do that. Without any real reference elsewhere, you can use those abilities. Teleport is a word everyone can understand. Change shape might be a better word than Polymorph. But they don't have to reference spells if they aren't complicated.
And periodically if you wanted a complex monster, there should be an optional rule that says "You can add spells to a monster if you want, but it counts as being X levels higher" or whatever.
I just don't want to go back to having to look up spells every combat.
That's a bit orthogonal to what I proposed, but having a more ambiguous description in the monster and a detailed one in the spell description is fine, IMO: freedom of movement (moves normally despite impediments; PHB ZZZ), with the PHB entry explaining what that actually means.
That part of my post was less directed at you and more a comment on the information we have on 5e. I'm afraid that they may just go with ambiguous descriptions inside the PHB itself.
However, having a ambiguous description in the monster's description is probably just as bad as having no description. If I want to know if Freedom of Movement can stop Entangle, I'm still going to look up the full description of the spell anyways since "impediments" can mean a lot of things.