AD&D Monster Manual part 55

TREANT: Treants first appeared in D&D Vol. 2: Monster & Treasure, originally under the name of "Ents".  They've grown more powerful statistically since then (see below), but otherwise remain much the same, with their main power being the ability to animate normal trees and cause them to attack.  The one major change is that they now have a greater vulnerability to fire.

The opening line of the Treant entry reads "Treants are strangely related to humans and trees, combining features of both species".  I'm not sure how literally I should interpret them being related.  If I wanted to tie it in I could say that the animating power behind each Treant is a human spirit, but I think I prefer to keep them as ancient and mysterious. 

Stat Changes:
Number Appearing: Old - 2-20, New - 1-20; Armor Class: Old - 2, New - 0; Movement: Old - 6", New - 12"; Hit Dice: Old - 8, New - 7 to 12;

TRITON: Tritons first appeared in Supplement I: Greyhawk.  They've been severely depowered; in the original version, all Tritons had spellcasting abilities, and they had around double the amount of Hit Points.  They still retain their 90% magic resistance, though, and if enough Tritons are encountered there'll be high-level fighters, clerics and magic-users.  They can even use psionics, as they were able to in Supplement III: Eldritch Wizardry.  They still ride sea horses, and have gained hippocampi and sea lions as pets.

Tritons wear armour made of scales that gives them AC 4, and their primary weapons are swords, daggers, spears, tridents and crossbows.  Triton leaders usually have a conch shell that has a number of magical powers.  The shell, when blown, can calm rough waters; summon hippocampi, sea lions or sea horses; and cause marine creatures of animal intelligence to flee.  It's not stated, but I'd rule these shells as being unusable by any but a Triton.

Triton society is fleshed out a little bit more than it had been.  They're rumoured to be from the Elemental Plane of Water, sent to the material plane for a purpose unknown to man.  They worship the god Triton, and have fought wars against ixitxachitl, koalinths, lacedons and most frequently the sahuagin.  Surprisingly, they like humans.  It's a rarity in the Monster Manual.

Stat Changes:
Number Appearing: Old - 5-30+, New - 10-60; Armor Class: 6 to 4, New - 5; Hit Dice: Old - 5 to 7, New - 3; Damage: Old - 3-18, New - By weapon type