From Bulbapedia, the community-driven Pokémon encyclopedia.
The RageCandyBar (Japanese: いかりまんじゅう Rage Manjū) is an item available only in Pokémon Gold, Silver, Crystal, HeartGold, and SoulSilver. Despite this, it has been mentioned and has been referenced in other games.
In the games
Gold, Silver, and Crystal
The RageCandyBar (spelled without spaces due to character limitations) first appeared in Generation II. It is a local specialty of Mahogany Town, in the same vein as the Lavaridge Town's Lava Cookie is in later generations. The first place that the item can be bought is from a salesman blocking the way out of Mahogany (who is there to prevent the player from skipping the Lake of Rage/Team Rocket HQ event); he sells these for 300. Once the Red Gyarados has been captured or defeated, however, he will sell no more. The RageCandyBar remains unavailable until the Team Rocket Executives have been defeated at the Radio Tower. After this, it can be purchased from the Mahogany Town Shop for, again, 300.
In these games, the RageCandyBar acts like a Potion: healing 20 HP. Unlike future games, it is a regular item, and can be held by Pokémon and traded between games.
FireRed, LeafGreen, and Platinum
Though the item does not exist in the coding of the Generation III and Sinnoh-based Generation IV games, it is mentioned repeatedly. In Two Island in Pokémon FireRed and LeafGreen, it is said to be a favorite of Bruno of the Kanto Elite Four, who left the island disappointed when he found that the shop was sold out of them. No mention is made in Pokémon Diamond and Pearl, however, this is changed in Pokémon Platinum, where it is claimed to be sold out in the Veilstone Department Store basement. It is also said to be favored by Professor Rowan, who can be found occasionally in the basement of the Veilstone Department Store sulking about the fact they've run out of them.
In both mentions, it is spelled with spaces in its name, unlike in Generation II: FireRed and LeafGreen spell it as "Rage Candybar" (strangely not in all-caps, unlike other items), while Platinum names it as "Rage Candy Bar".
In HeartGold and SoulSilver
The RageCandyBar appears in a similar capacity in HeartGold and SoulSilver as it did in Generation II. It is available in Mahogany Town only, but this time is a key item. This prevents the player from trading it to Pokémon Diamond, Pearl, and Platinum, where data for the item does not exist. The RageCandyBar is not a healing item in these games, however, and may be exchanged to a non-player character in the Underground Path (Routes 5-6) in Kanto for TM64 (Explosion).
In this version of the game, the man who blocks your way selling the RageCandyBar will still be selling it after the Team Rocket invasion (instead of the lady in the shop). However, he will refuse to sell you more than one.
Spelling between generations
Each time it is mentioned, the spelling of the item's name is slightly different. In Generation II, it is spelled in all-caps with no spaces in the item list, as "RAGECANDYBAR". FireRed and LeafGreen add a space and capitalize normally, noting it as "Rage Candybar". Platinum, finally, puts spaces between all words as "Rage Candy Bar". In its return in HeartGold and SoulSilver, it is spelled as RageCandyBar, due to the twelve-character limit and capitalization change.
The RageCandyBar 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 locales (much as the RageCandyBar 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.