In Generation II, the item's name is spelled in all caps and with no spaces, thus rendering it as "Ragecandybar".
When referred to in Pokémon FireRed and LeafGreen, it is instead rendered as "Rage Candybar" as it is mentioned exclusively in dialogue due to it not being an item in these games. Likewise in Pokémon Platinum, the item is only specified in dialogue but its name is rendered normally as "Rage Candy Bar".
In Out-Odding Oddish, while on his way to the Pokéathlon Dome, Gold was seen eating some Rage Candy Bars that he had recently purchased. He also threw one into Whitney's mouth, almost causing her to choke. Gold later gave a Rage Candy Bar to Bruno, who immediately fell in love with its delicious taste. In All About Arceus IX, Whitney, who was shown to have a crush on Bruno, was seen sending the Elite Four member a boxful of Rage Candy Bars, much to his pleasure.
The Rage Candy Bar is based on the Japanese manjū, a type of pounded rice cake. Manjū are sold in Japan as confectionery, and are often unique to certain places (much as the Rage Candy Bar is to Mahogany Town). A direct translation would thus prove confusing for English-speaking players, providing reasoning behind naming it as a candy bar. Its in-Bag sprite depicts it in a small box, which could pass for a candy bar wrapper, preventing revelation of the translation issue.
According to Nob Ogasawara, under his alias of Douglas Dinsdale (also Doug Dinsdale), in a thread apparently archived from the Something Awful forums and available at the Let's Play Archive, the name Rage Candy Bar is a mistranslation that was decided by his handlers at Nintendo of America. He claims that the correct translation of いかりまんじゅう is "Rage Dumpling" or, given the twelve-character limit at the time, "Rage Bun". The name Cake of Rage was also suggested by him for the English localization, which would match the Lake of Rage.
In Pokémon Platinum, it is supposed to be sold at Veilstone Department Store's basement, but the vendor selling them always says they're sold out of them. They are also said to be favored by Professor Rowan, who can occasionally be found in the Department Store basement, sulking about the fact that the store has run out of them. An NPC in the basement claims to be the one who has bought all the available Rage Candy Bars.
While it is obtained as a Key Item in Pokémon HeartGold and SoulSilver, the Rage Candy Bar is still required to be bought for 300. Due to this, it is thus far the only Key Item available for purchase.