Super Mario series
Super Mario Bros. / Super Mario Bros.: The Lost Levels
Mario has to fight seven Bowser Impostors (one in each castle) before he can reach Bowser's Castle and fight the real one. Hitting a Bowser Impostor with fireballs reveals that they are really just common enemies disguised as their king with a spell (presumably put on them by Bowser). Some of these enemies are colored light gray (because of the castle's gray color palette), however. The enemies and their attributes are:
Super Mario Bros.: The Lost Levels additional worlds:
In Super Mario Bros.: The Lost Levels, Bowser Impostors also appear in Worlds 8-4, 9-3, and D-4, along with the real Bowser. Unlike the other Bowser Impostors, these Bowsers have more of a mint-green body with a brown muzzle instead of the normal color. This type of Bowser is a magical copy of the real one; it also breathes fire and throws hammers in all levels encountered except in World D-4, where it is a Spiny which breathes fire, just like the one at the end of the level.
Super Mario All-Stars
In Super Mario All-Stars, they also lack their alternate coloration and look identical to Bowser. Because of this and the fact that this Bowser is the only Bowser that appears in World 9, players unaware of the original version of the game have mistaken them for the real Bowser. However, one means of distinguishing them from the real Bowser is the music that plays; the real Bowser's music is different.
On a side note, all Bowser Impostors in Worlds 9-D only throw hammers in the Super Mario All-Stars remake. Strangely, the real Bowser in World D-4 also only uses hammers, unlike in World 8-4, where he uses hammers and spits fire.
Super Mario Advance 4: Super Mario Bros. 3
In Super Mario Advance 4: Super Mario Bros. 3, a Bowser Impostor appears as a boss in the e-world level "Classic World 1-4", where it plays a similar role to that played in Super Mario Bros., though it looks identical to the game's real Bowser. It is revealed that the Bowser Impostor is a Goomba after being defeated.
Super Mario 3D Land
Bowser Impostors appear in the Nintendo 3DS game, Super Mario 3D Land. They have Tanooki tails like Tanooki Mario, and they attack in a similar way as in Super Mario Bros., breathing several giant fireballs from a far place and jumping around the stage.
However, unlike in Super Mario Bros., only two Bowser Impostors appear: one in World 1-Castle and the other in World 5-Castle. They are also immune to Fire Mario's fireballs and any other projectiles. In order to defeat a Bowser Impostor, Mario or Luigi has to press a switch at the end of bridge to make it collapse and send the Bowser Impostor into the lava. After falling into the lava, Bowser Impostors bounce upwards and show their true forms with a Super Leaf beside them. The Bowser Impostor in World 1 is actually a Goomba, while the one in World 5 was a Magikoopa.
Mario & Luigi: Superstar Saga
A Bowser Impostor makes a cameo appearance in Mario & Luigi: Superstar Saga. It is seen in a special area inside Bowser's Castle, which is a side-scrolling level imitating a Super Mario Bros. level, and appears at the end of that level. The Bowser Impostor appears to be made of cardboard, and Mario must run underneath him and hit the axe to make him fall.
Mario & Sonic at the Sochi 2014 Olympic Winter Games
A Bowser Impostor appears as a boss in Mario & Sonic at the Sochi 2014 Olympic Winter Games, specifically in the Mario version of Dream Figure Skating. The appearances of the stage and its boss are based off Super Mario 3D Land. He chases the team across a giant collapsing bridge, spitting fireballs at them. The team must hits four buttons at the same time to cause the Bowser Impostor to fall into lava, where he turns into a Goomba and a Super Leaf.
Super Smash Bros. for Nintendo 3DS / Wii U
According to one of the randomly generated tips in Super Smash Bros. for Wii U, Bowser's eighth alternate coloration in these games is based upon the Bowser Impostor that appears in World 8-4 of Super Mario Bros.: The Lost Levels.