Bowser's Brother

Bowser's Brother is a character similar to Bowser who appears in Super Mario Bros.: The Lost Levels. His identity varies in supplemental material, with the only consistent characteristic being his bluish color that was removed in Super Mario All-Stars.

Super Mario Bros.: The Lost Levels
Bowser's Brother makes his first appearance in the Family Computer Disk System version of Super Mario Bros.: The Lost Levels. There, he appears in World 8-4 and World 9-3. He tries to stop Mario or Luigi both by throwing hammers at them and by breathing fire.

While distinguished from Bowser through the color of his skin and shell (brown and deep cyan, respectively), when defeated with fireballs, he does not turn into one of Bowser's minions, revealing him to be distinct from the fake Bowsers. In World D-4, a fake version of him appears that only breathes fire and, when defeated with fireballs, is revealed to be a Spiny.

Due to All Night Nippon Super Mario Bros.'s version of World 8-4 being reused from The Lost Levels, Bowser's Brother also returns in that game.

In the Super Mario All-Stars version, Bowser's Brother is recolored to look identical to Bowser and also appears in D-4 in place of the fake version of him. However, there is an unused palette in the game's data meant for him.

Super Mario Bros. Deluxe
Bowser's Brother returns in Super Mario Bros. Deluxe. Here, he appears in the game's port of Super Mario Bros.: The Lost Levels (referred to in this game as "Super Mario Bros. For Super Players"), having the same role and appearance as the original game.

Super Smash Bros. series
In Super Smash Bros. for Nintendo 3DS / Wii U and Super Smash Bros. Ultimate, the eighth recolor of Bowser is said to be him in the tips. It replaces a similar-looking recolor from previous games that had yellow hair and green limbs, likely based off Bowser's original artwork (though with the shell and skin colors swapped).

Blue-colored versions of Bowser distinct from Bowser’s Brother
Even though these blue-colored versions of Bowser likely affected the depiction of Bowser’s Brother, they are not stated to be him.

Super Mario Bros.
In the original Super Mario Bros., the palette used for castle sections before the final boss battle causes an unintended consequence in which Bowser has the same blue coloration of Bowser’s Brother. This is due to color of Bowser's skin and shell being obtained through a palette swap commonly used in castle sections of the game, except for the parts in which the boss is fought. Because of this, standing near the end of a castle in a way so that Bowser is barely seen on the right-hand side of the screen causes him to appear dark shades of teal, but he will revert back to his correct colors if Mario or Luigi advances any further. The same graphical oversight can be seen in Super Mario Bros.: The Lost Levels. Additionally, defeating a Fake Bowser before his color change will result in the enemy acting as his true form to retain this color scheme (such as World 5's Fake Bowser becoming a Lakitu using the underground/castle color scheme).

Mario Golf (Nintendo 64)
The second palette swap of Bowser in the Nintendo 64 version of Mario Golf features a blue skin and a blue shell. Contrary to what would happen in Super Smash Bros. for Wii U, the shell and head have a lighter blue color than the skin.

Mario Tennis (Nintendo 64)
In the Nintendo 64 version of Mario Tennis, one of Bowser's palette swaps features the same blue colors seen in Mario Golf.

Identity
Initially mentioned as just another Bowser in the Japanese guides of Super Mario Bros.: The Lost Levels, without specifying what was his relationship to Bowser besides not being his twin, in the character books of the '90s released before the release of Super Mario All-Stars the blue Bowser is described as being Bowser's younger brother,  with Super Mario Zen Hyakka, a book written in collaboration with Nintendo, putting him as part of the 「王族 」 (Ō-zoku, literally "King Tribe") and thus a leader of the Koopa Troop and member of Bowser's royal family, along with the Koopalings and Bowser himself. Due to his recoloring from blue to green in Super Mario All-Stars, he is not considered a distinct character from fake Bowsers in the Japanese official guide book. However, he is called "Bowser's Brother" and stated to be his twin in the Nintendo Power guide for Super Mario All-Stars. After the release of Super Mario All-Stars, he started being considered a fake Bowser, with the exception being the Prima Games guide for Super Mario Bros. Deluxe, where he is again considered Bowser's brother. In Super Smash Bros. for Wii U, the English text suggests the blue Bowser is a fake while the Japanese version leaves his nature ambiguous. Super Mario Pia, a memorial book also written in collaboration with Nintendo, depicts the blue Bowser as a particular fake Bowser. Encyclopedia Super Mario Bros., which was released around the same time, distinguishes him from fake Bowsers but states that his identity is unknown.

Super Smash Bros. for Wii U / Super Smash Bros. Ultimate tip

 * "A Bowser Impostor?! In Super Mario Bros.: The Lost Levels, Mario fights a blue fake Bowser before the real thing. That blue fake is his eighth color in this game!"

Trivia

 * Due to his bluish coloration, Bowser's Brother resembles Shigeru Miyamoto's original blue depiction of Bowser in artwork for Super Mario Bros., prior to the character's redesign in artwork for Super Mario Bros.: The Lost Levels.