AD&D Monster Manual part 52

SU-MONSTER: These creepy psychic monkeys first appeared in Supplement III: Eldritch Wizardry.  They've changed little, even using much of the same wording from the original entry.  As before they have a latent psychic ability, and can lash out with a randomly determined psychic attack is psionics are used within a certain range of them (psychic crush, psionic blast or mind thrust).  They also retain the same likelihood to appear as a family unit, with a father, mother and children.  If the children are attacked the mother fights at double value, and if the mother is attacked the father does the same.

Su-monsters were originally described as being "highly evil", but here their alignment is given simply as Chaotic.  This is odd for the Monster Manual, as most of the alignments do have the good-evil axis specified.  I think I'll fudge this one, and continue to read them as Chaotic Evil.  The only other change is that they're bit tougher, with an extra Hit Die and higher damage on their attacks.  Allow me to continue using the same explanation I've been leaning on too much lately: "these are the full-grown adults, and the older ones were not quite fully mature".

Stat Changes:
Hit Dice: Old - 4+2, New - 5+5; Damage: Old: 1-3 per claw & 1-8 per bite, New - 1-4 per claw & 2-8 per bite

SYLPH: Sylphs, appearing here for the first time in D&D, are a sort of aerial nymph.  They can cast spells as though they are 7th-level, although it's not specified whether they cast as clerics or magic-users.  Cleric spells seem to me to be more fitting.  They can also turn invisible at will, and conjure an air elemental once per week.  The one bit of personality they are given is that they have a 20% chance to befriend a good-aligned character and help them out in some way.

THOUGHT-EATER:  Brrrrr.  Thought-eaters give me the willies.  They appear here for the first time.  Thought-eaters dwell in the Ethereal Plane, but they're able to sense any psionic energy used nearby.  Once a thought-eater is close it will consume any spell or psionic ability that its victim uses; it will be sated once it has eaten from 101-200 points.  If the thought-eater gets even closer it can begin feeding on the victim's Intelligence score, resulting in permanent stat loss.  The rate of such feeding isn't specified, but I'd be inclined to make it 1 point per round, or perhaps an even slower rate.  They can only be killed by other ethereal characters, but spells such as mind blank and other defenses against psionics will be effective at keeping them away.