Longchu is the result of swapping the moveset of Ganondorf with that of Pikachu in the Wii game Super Smash Bros. Brawl. This can only be done by a haxxor.
In the months before the release of Super Smash Bros. Brawl in the US, MeleeKirby had a podcast in which people could watch him play his Japanese copy of the game. One day he started to screw around with his disc and was able to switch the movesets of certain characters. Because the game could not handle this level of haxs, abominations of these characters were born. When pikachu was given the moveset of ganondorf, the result bore a striking resemblance to longcat, as was pointed out by tecnic22 in the chatbox next to the podcast. It is most commonly used in exclamations when referring to said character.
Other nicknames for hilarious-looking moveset swaps are Crab Link (Toon Link w/ Metaknight's moveset) and Mr. Game and Whatthefuck (Mr. Game and Watch w/ R.O.B.'s moveset).
tecnic22: LONGCHU IS LOOOOOOOOOOOONG!!!