It is not known to evolve into or from any other Pokémon.
Along with Raikou and Entei, it is one of the Legendary beasts resurrected by Ho-Oh after the burning of the Brass Tower. Of the three Legendary beasts, Suicune is said to represent the rains that quenched the flames of the burning Brass Tower.
Suicune is a slim, quadruped, blue, mammalian Pokémon with white, diamond-shaped markings. Its face and underside are white as well. Suicune has a thick, purple mane that resembles the aurora borealis and two white, streamer-like tails that wave forward. It has a long, white snout. Suicune has a large, hexagonal, cerulean blue crest on its forehead that resembles the antlers of a Qilin with two prongs on either side of the base. It has red eyes, a small tuft of fur under its chin, and small paws with white paw pads.
Suicune has the power to walk across water and purify dirty water with one touch. It travels across grasslands in search of water to purify. Northern winds always seem to blow stronger in its presence. In Pokémon Crystal, Suicune is said to be the closest of the Legendary beasts to Ho-Oh. It is also said to have a link to the Unown. In the anime, Suicune is shown to be heroic, serious, and calm during dangerous situations.
Suicune appeared as a beam of light in Slugging It Out With Slugma. It wasn't officially seen until Querulous Qwilfish where it had cured the baby Qwilfish Crystal had rescued and cleansed the sea water. It appeared in Really Raikou & Entirely Entei I, joining Raikou and Entei against Falkner and afterwards discussed with them about Ho-oh. It was revealed in a flashback in Hello, Lickitung, where it was awakened by Yellow along with its two counterparts, and the three raced around Kanto and Johto looking for Trainers to help them defeat the Masked Man to free Ho-Oh. Suicune, in particular, personally challenges many of the Gym Leaders in the two regions. During its travels, it encounters both Eusine and Crystal, both of which become obsessed with capturing it. Eusine acquires the Crystal Bell from the Rocket trio he defeats, but he and Crystal are still unable to capture Suicune. Crystal has her confidence shattered by this (and by the guilt of interrupting Suicune's attempt to contact its old master) and has to retreat back to her old training grounds to regain her confidence, which she does by reacquiring her catching skills capturing a wild Larvitar. Suicune later chooses Misty as its partner and fights alongside her, but then pairs up with Crystal anyways after Misty becomes injured (much to the dismay of Eusine).
A Ditto transformed into a Suicune to distract Crystal and Eusine in Miltank Melee, while Ken, Al and Harry surrounded the real one. Ditto's stretchiness was able to grab both of them, but Crystal managed to escaped with her Monlee and defeated the Ditto.
Suicune reappears again in Super Smash Bros. Ultimate, functioning the same as in previous games. In these games, it can also be summoned from a Master Ball.
Melee trophy information
This mystical aurora Pokémon constantly journeys in search of places with pure flowing water. Suicune wields the power to instantly purify any contaminated water it finds. Some say Suicune is the north wind reborn. Its strange and dramatic story is told over the course of the Crystal version of the Pokémon game.
Brawl trophy information
"An Aurora Pokémon. It has the power to instantly purify even the filthiest water. Suicune is said to be the reincarnation of the northern wind, so when it shows up, so does that wind. Suicune uses the move Aurora Beam--aurora light focused into a powerful beam. Along with inflicting damage, this attack also lowers the target's attack power."
3DS/Wii U trophy information
NA: A Legendary Pokémon that emerged from the ashes of the Burned Tower, Suicune is a Water-type Pokémon easily identified by its long, purple mane. In battle, it'll head toward a nearby fighter and fire an Aurora Beam all the way to the edge of the screen. If you get hit, you'll be frozen solid!
PAL: Suicune is one of the Legendary Pokémon from the Burned Tower in Ecruteak City. Its mane is said to resemble an aurora floating over it. If summoned in battle, it'll make its way straight for a nearby fighter and unleash its Aurora Beam. The target, frozen in place, is unable to resist as it's blown off the stage.
Pokémon Crystal: It is possible to see Suicune multiple times on the overworld before the actual battle in Bell Tower. In these occasions, it will always hop away on being spotted.
Suicune is thought to be based on numerous creatures and mythological deities, mainly the Qilin or "Kirin", an Eastern legend that has traits of chinese dragons, unicorns, deer, big cats, and other beasts. The Qilin was said to be able to walk on water and is known to be a powerful and peaceful creature. The crest on Suicune's head roughly matches the description of the beast's antlers. The spots of Suicune are also significant, seeming to combine the spots of a leopard, taking homage from Fūjin, the Shinto god of The North Wind, who wears leopard skin. Leopards themselves are found in Asia and are featured in myths that show it is a widely respected sacred animal like the Qilin. The way it runs and roars in the anime may also relate to the leopard. Being a Pokémon that represents the wind and rain, Suicune may also be based on some of the world's numerous myths of wind deities. Suicune's mane is meant to resemble a northern aurora.
Suicune may be a combination of 水 sui (water) and 君 kun (monarch). The game title it is the mascot of suggests that it may derive from 水晶 suishō (crystal).