It is not known to evolve into or from any other Pokémon. However, as of Generation III, Unown has 28 derivative shapes. There are 26 English alphabet characters, plus two punctuation marks: ? (question mark) and ! (exclamation mark). Its shape is determined by its personality value.
They tend to stick to walls or orbit other Unown when in a group.
Unown are floating, flat, eye-shaped Pokémon with appendages that make them resemble letters. They are black all over.
Unown's only known move in battle is the enigmatic Hidden Power.
In the anime, they are capable of reading the thoughts of other lifeforms. Great numbers of them are capable of warping reality to suit the dreams and wishes of whoever they read the thoughts of and is in control of the Unown.
Their mind reading ability seems to suggest that they should be able to learn Mind Reader. However, they can only do this when more Unown are present, so it could be a shared effort rather than an ability unique to each Unown.
Unown can make telepathic contact with humans (and many other creatures also). Unown have a timidness that rarely makes them venture outside of their dimension.
Unown play a large role in the third movie, creating the illusions which drive the plot.
Notes about Unown can be seen on Professor Oak's computer at the beginning of the movie. The information (as presented in the Japanese version) has been cleaned up and transcribed here:
Because of their very timid and careful character, Unown do not want to go out to the external world.
Unown have the power to perceive the feelings of other creatures (including human beings).
Unown communicate with each other over electromagnetic waves but cannot use telepathy.
Given all this, Unown are still classified as a species of Pokémon, as opposed to any other sort of creature.
An Unown fell out of the Unown dimension in Address Unown. After it was healed by the group, it helped defend against Team Rocket and its mind merged with that of Ash's Larvitar so that the group could help it to address its inner pain.
Unown later appear in Volume 24, having been awakened by Sird from the Tanoby Ruins to use as a distraction while she fought Lorelei. The Unown later appear again throughout the volume, being employed by Team Rocket as living security cameras while impeding the progress of Red, Blue and Green.
It also appeared in DP saga when Diamond, Pearl and Platinum were visiting Solaceon Town. Diamond and Pearl befriended two Unown (D and P) who behaved like them.
When one Unown is released from a Poké Ball it will fly off the screen. It then returns with a large swarm similar to what Beedrill did in the original Super Smash Bros. The only difference is that Beedrill only attack from left or right while Unown can come from any direction. Any opponent caught in this is damaged and juggled.
There are also a number of Unown Pokéfloats. In the second half of the cycle, a number fly across the screen. Towards the end of the cycle, staying on the stage relies on hopping between the Unown.
These Psychic-type symbol Pokémon resemble ancient hieroglyphics found on rocks, a resemblance that's said to be more than superficial. Quite a few different shapes of Unown have been confirmed, and it's believed that each type has unique abilities. These enigmatic Pokémon can be found in the Ruins of Alph.
This Pokémon was unavailable prior to Generation II.
In Spell of the Unown, Professor Oak had some pictures of unreleased Unown on his computer. They resemble a few Greek alphabet letters, especially σ (lowercase sigma), Σ (capital sigma), Π (pi) and Ω (omega). However, lowercase letters have never been made into Unown before, so this may be an error or a misinterpretation by fans. The top-middle Unown may be also a variation of an ancient Egyptian symbol of protection, the Eye of Horus, or the Russian letter Я (ya). This may lead to the conclusion that there are more forms of Unown, being the prototypes of other known (or unknown) alphabets.
Unown apparently generate a radio signal where they are found in large numbers, which can be listened to by any Trainer with a PokéGear.
It is the first non-legendary to be in a central role in a Pokémon movie.
Unown, Nidorina, Nidoqueen, and Ditto are the only non-legendary, non-baby Pokémon that cannot breed with Ditto.
Despite being in the book of legendary Pokémon, Unown is not considered a legendary Pokémon.
The two punctuation Unown have half-closed eyes; all alphabetic Unown have fully open eyes.
If a player does not have Unown's Pokédex data on a Generation II game when in Oak's lab in Pokémon Stadium 2, Unown's Pokédex number is listed as 252 instead of 201. This happens regardless of which Pokédex display mode is selected.
It has the smallest movepool of all Pokémon, learning only one move, Hidden Power, and being unable to learn any other moves through TMs, HMs, and Pokémon breeding (as it cannot breed). It shares this distinction with Ditto, and Smeargle, however they can potentially expand their movepools through the use of Transform and Sketch respectively.
Despite having their own movie about their power, they are relatively weak in the games due to a very shallow move pool.
Their power of warping reality is barely acknowledged in the games. Only the pokédex data in Pokémon Platinum even hints to their power, saying that "an odd power is said to emerge" when two or more are together. Nothing special occurs when two Unown are sent out side by side in a double battle, however.
Generation II is the only Generation in which all Unown have their own separate animations. Consequently, Unown ? (question mark) and ! (exclamation mark) never experience this because in all following generations, the Unown share the same animations.
Their appearances are corruptions of the modern Latin alphabet. The shapes of the Unown are used like runes in the Pokémon world, although in Generation III their place has been taken by Braille.
Unown's name is a corruption of the word unknown.
In other languages
German: Icognito - From Inkognito (incognito); similar to the English corruption of "unknown".
French: Zarbi - From the word bizarre in verlan, a youth language.
Norwegian: Ukjent - Unknown, only in episode 264 - Address Unown, where it was mistakenly translated.
Swedish: Okänd - Unknown, only in episode 264 - Address Unown, where it was mistakenly translated. Also of note was that they always referred to the Unown with very bad grammar, for example: "the place where Unknown comes from".