The Couatl is pretty crazy if you think about it: half bird, half snake, genius intelligence creatures of lawful good who are worshipped as gods in the rainforests. It is not hard to surmise that this beast was based on the Mayan/Aztec god Quetzlcoatl. Clearly, any Meso-American campaign would benefit from the presence of Couatl.
I think most depictions of the Couatl over-emphasize its "serpent-ness". By contrast, I would tend to emphasize its "bird-ness", which highlights its 'lawful goodness' and essential friendliness. The 1st ed. classic picture of it does it no favors, emphasizing that wicked-looking snake eye staring right at you, with slithering tongue and sharp teeth, with its scaley body dropping off behind it. I imagine that if it kept its mouth shut, having large wings and covered with colorful feathers, it could easily pass for a bird with a long body.
Keep in mind, though, because they can polymorph self at will, they will most likely NOT be encountered in their natural form. To humans, they would no doubt appear as a "feather-plumed" human, with spooky-looking snake eyes and an aloof personality. If it came down to combat, we would get one of those dramatic moments when they would suddenly assume their true form before opening the can of whoop ass. They could also assume their true form in the context of the culmination of a religious ritual. Whatever the case, their polymorph self power should provide some good dramatic role-playing fodder.
The 1e MM description of its psionic powers in vague, basically leaving it up to your creativity, suggesting only 2 major/4 minor disciplines with "commensurate attack and defense modes". They are definitely NOT psionically overpowered, with one of the lowest psionic ability points of any psionic god-like creatures out there. However, consulting the relevant table in Appendix I, we see that the combination of 4 minor/2 major disciplines is actually at the top of the chart. "Commensurate" attack/defense abilities, if we follow the "top of the chart" logic, would imply they have them all, and as that makes the whole thing much easier, I'm just going to go with that.
As for filling out their power package of psionic disciplines, given that they are often worshipped by the locals, we can imagine a fairly typical and appropriate set. Powers like Cell Adjustment (healing/cure disease), Object Reading, Precognition, and Molecular Rearrangement (transmute metals) would be especially likely to draw human worshippers.
Psionic Ability: 60-110 (50 + 10d6)
Psionic attacks: All
Psionic defenses: All
Etherealness, at will
Polymorph self, at will
Spell use: d100: 01-45 --> 5th level magic user, 46-80 --> 7th level cleric 35%, 81-00 --> both
[Typical clerical package: Bless, Protection from Evil, Create Water, Know Alignment, Hold Person, Silence 15' radius, Dispel Magic, Remove Curse, Neutralize Poison
Typical magic user package: Sleep, Charm Person, Affect Normal Fires, Identify, Detect Invisibility, Locate Object, Monster Summoning 1]
4 minor disciplines (suggested choose from): Cell Adjustment (healing/cure disease), Clairaudience/Clairvoyance, Detection of Good/Evil, Object Reading, Precognition, Sensitivity to Psychic Impressions
2 major disciplines (suggested choose from): Telekinesis, Energy Control, Molecular Rearrangement (transmute metals), Molecular Manipulation (weaken objects)
The Other Psionic Monster starting with C
The Crysmal (of the 1e MM2) is listed as having psionic powers, as well. As some sort of weird sentient crystalline creature, sure why not! Like some of the "sentient plants" that develop psionic powers, it could definitely throw your players for a loop.