(from 5th Edition Monster Manual - 2014)

Couatls are benevolent serpentine beings of great intellect and insight. Their brilliantly colored wings and gentle manner speak to their celestial origins.

Divine Caretakers. Couatls were created as guardians and caretakers by a benevolent god not worshiped since the dawn of time, and which is forgotten now by all but the couatls themselves. Most of the divine mandates given to these beings are long since fulfilled or failed. However, a number of couatls still watch over ancient power, await fulfillment of prophecy, or safeguard the heirs of creatures they once guided and protected. Regardless of a couatl’s task, it prefers to remain hidden, revealing itself only as a last resort.

Truth Tellers. A couatl can’t lie, but it can withhold information, answer questions vaguely, or allow others to jump to the wrong conclusions if doing so is necessary to protect something, to keep promises, or to hide the secret of its existence.

Ancient and Few. A couatl can live for ages without sustenance, even surviving without air, but these creatures can die of disease or the passage of time. A couatl can sense its end up to a century beforehand, but it has no insight into the manner of its demise.

If a couatl has already accomplished what it set out to do, it accepts its fate. However, if its imminent death endangers the completion of its goals, it actively seeks out another couatl with which to produce offspring.

The mating ritual of couatls is a beautiful and elaborate dance of magic and light, which results in a gem-like egg from which a new couatl hatches. The parent that sought out the mate raises the newborn couatl and instructs it as to its duties, so that it can complete whatever task the parent leaves unfinished.

(from Monster Manual v3.5 - 2003):

This great serpent with rainbow-feathered wings appears confident, powerful, and totally aware of everything around it. The couatl is legendary for its sheer beauty, vast magical powers, and unwavering virtue. Its intelligence and goodness have made it an object of reverence in the regions it inhabits. 

A couatl is about 12 feet long, with a wingspan of about 15 feet. It weighs about 1,800 pounds. 

Couatls speak Celestial, Common, and Draconic, and also have the power of telepathy (see below). 

Combat: A couatl seldom attacks without provocation, though it always attacks evildoers caught redhanded. It uses its detect thoughts ability on any creature that arouses its suspicions. Since it is highly intelligent, a couatl usually casts spells from a distance before closing. If more than one couatl is involved, they discuss their strategy before a battle. 

Constrict (Ex): A couatl deals 2d8+6 points of damage with a successful grapple check. 

Improved Grab (Ex): To use this ability, a couatl must hit a creature of up to two size categories larger with its bite attack. It can then attempt to start a grapple as a free action without provoking an attack of opportunity. If it wins the grapple check, it establishes a hold and can constrict. 

Poison (Ex): Injury, Fortitude DC 16, initial damage 2d4 Str, secondary damage 4d4 Str. The save DC is Constitution-based. Psionics (Sp): At will—detect chaos, detect evil, detect good, detect law, detect thoughts (DC 15), invisibility, plane shift (DC 20), polymorph (self only). Effective caster level 9th. The save DCs are Charisma-based.

Spells: A couatl casts spells as a 9th-level sorcerer. It can choose its spells known from the sorcerer list, the cleric list, and from the lists for the Air, Good, and Law domains. The cleric spells and domain spells are considered arcane spells for a couatl, meaning that the creature does not need a divine focus to cast them. 

Typical Spells Known (6/7/7/7/4; save DC 13 + spell level): 0—cure minor wounds, daze, disrupt undead, light, obscuring mist, ray of frost, read magic, resistance; 1st—endure elements, mage armor, protection from chaos, true strike, wind wall; 2nd—cure moderate wounds, eagle’s splendor, scorching ray, silence; 3rd—gaseous form, magic circle against evil, summon monster III; 4th— charm monster, freedom of movement. 

Ethereal Jaunt (Su): This ability works like the ethereal jaunt spell (caster level 16th). 

Telepathy (Su): A couatl can communicate telepathically with any creature within 90 feet that has an Intelligence score. The creature can respond to the couatl if it wishes—no common language is needed.

(from 2nd Edition Monstrous Manual - 1993):

The couatl are feathered serpents of myth and lore. It is believed that they are distant relatives of dragons, although this remains unproven.

So rare as to be considered almost legendary, the couatl is one of the most beautiful creatures in existence. It has the body of a long serpent and feathered wings the color of the rainbow. Since it has the ability to polymorph, the couatl will sometimes appear in the form of other creatures (always of good alignment).

Couatl are able to communicate via telepathy with almost any intelligent creature which they encounter. In addition, they can speak common and most serpent and avian languages.

Combat: A couatl will seldom attack without provocation, though it will always attack evildoers caught red-handed. Whenever possible, a couatl will attack from the air.

Since it is highly intelligent, the couatl will frequently use its spells from a distance before closing with its foes. If more than one couatl is involved, they will discuss their strategy before a battle. The couatl will also not hesitate to polymorph into another, more effective form in combat.

The couatl have a variety of abilities which make them more than a match for most other creatures. In addition to being able to polymorph themselves at will, a couatl can use magic. Fully 45% will be 5th level wizards, while 35% can act as 7th level priests. The remaining 20% are able to use both types of abilities.

In addition to their other magical abilities, couatl can render themselves and up to 450 pounds of additional matter ethereal at will. Further, they can detect good/evil, detect magic, turn invisible, and employ ESP whenever they desire to do so. The oldest and most powerful of couatl can also use a plane shift on themselves and up to 8 others. This ability has a 90% chance of reaching the desired plane.

The couatl uses its poisonous bite and constriction when forced into melee combat. When it bites it does 1-3 points of damage and injects a deadly toxin. If the victim fails a save vs. poison it is killed instantly. If the constriction attack succeeds, the victim takes 2-8 points damage each round until it or the couatl is killed.

Habitat/Society: This winged serpent is native to warm, jungle-like regions but can also be found flying through the ether. Their intelligence and goodness have made them objects of reverence by the natives of the regions which they inhabit. Considered to be divine, there are many legends in which the couatl is the benefactor of mankind and the bestower of such precious gifts as agriculture and medicine. There are even shrines in certain areas dedicated to the couatl, and any who attack or harm a couatl are automatically viewed as the blackest of villains.

Although solitary in nature, couatl think of themselves as a single, extended clan. This clan is led by the oldest and wisest of their numbers but assembles only in dire emergencies.

Most couatl dwell alone, making their lairs in caves and abandoned buildings in remote, uninhabited regions. They hunt jungle animals for food once every fortnight or so. Many enjoy traveling, often undertaking long journeys of exploration.

On rare occasions, a pair will mate for life and establish a joint lair. Unlike many other reptiles, the couatl bear live young. Births are rare, averaging only one per couple each century. Both parents participate in the rearing and education of the single offspring, and will fight to the death if their child is threatened. Young couatl reach maturity in thirty or forty years and, though some will elect to remain with their parents for as long as a century, will eventually set off in search of the couatl’s never-ending quest for wisdom.

Intellectually curious, all couatl have vast stores of information and enjoy learning more. When one of them learns some new and fascinating fact he will inevitably set out in search of his brethren to share and discuss it.

Couatl can sometimes be persuaded to help good adventurers or give sound council. If they feel that they are being sought for frivolous reasons, they will simply fly away. They are not greedy and do not seek treasure for its own sake. Aid from a couatl may well take the form of a magical item from its hoard.

Ecology: The couatl usually reigns supreme in its jungle, having little to fear from most other monsters.