Well, they all do some sort of weird glitch, the most common I've experienced are turning ZEG into a toad, paralyzing him, casting spell animations with no effect, and making him take 9999 damage at intervals for no apparent reason, and then turning him back to normal.

And, they all don't show the summon animation. Just mess him up. At least, for me.

What monster only drops one item, a megalixir? (That I know of; GBA version.)