Physiology
Mew is normally described as a pink cat-like creature, though its morphology is actually more like a jerboa. Mew's hair is very fine and can only be seen under a microscope. Mew has big hind legs like a rabbit, and short arms with three-fingered paws. Mew has triangle-shaped ears and baby-blue eyes. Mew is said to have the DNA of every single Pokémon contained within its body. It is often depicted as being an embryo or fetus.
Gender differences
Mew is a genderless Pokémon. However, many incorrectly assume it is female due to its coloring.
Special abilities
Mew's main abilities include turning invisible at will, creating force-fields of psychic energy, and being able to transform into any one of the other 648 species of Pokémon. Mew is capable of wielding every type of attack possible. It also has the ability to float in mid-air, in much the same fashion as Celebi and Jirachi. It has been seen to be able to swim underwater as well.
Behavior
In the anime and manga, Mew is a very curious and intelligent Pokémon that will only appear to someone with a pure heart. It loves playing around and swimming, and is known to act very childish at times, wanting to play childish games with those around it. It is attracted to lullabies and will try to stay as close to music as possible. All in all, Mew is known as a happy, playful and optimistic Pokémon, unlike its clone, Mewtwo. In the games, Mew is very generic personality-wise, but still retains playfulness and needs to be chased down on Faraway Island before it can be captured.
Sightings In-Universe
Reports found in the Burned Mansion found in Cinnabar Island in the Kanto region speak of expeditions of scientists, either independent or Team Rocket-funded which had sighted Mew in Guyana, South America, the place where its first sighting was reported. However, since Mew can either make itself invisible or transform itself into any other Pokémon, very few people have knowingly seen it, and it is called a mirage by most. Since many scientists believe that Mew is the ancestor of all Pokémon, and a number of extraterrestrial Pokémon have been discovered, it is possible that Mew could inhabit other areas of the universe, and whether it originates from Earth at all is debatable. Whether Mew is a unique Pokémon or a species of Pokémon is currently unknown.