I know right he is my favorite of the newest batch I think but then I am always a sucker for the electric rodents!
There are a total of 8 Electric Rodent Pokemon over the course of 6 regions only 3 of them are an Evolutionary line.
Pichu is the pre-evolved form of Pikachu introduced in Johto's Gold, Silver, Crystal, HeartGold and SoulSilver games. When it has a high enough friendship it will evolve into a Pikachu from the original Kanto Red, Blue, Green and Yellow Games along with the remakes FireRed and LeafGreen. Just like in the original games you can evolve it to a Raichu by using a Thunderstone on it.
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In Ruby, Sapphire, and Emerald the Gen 3 games for Hoenn they introduced two new Electric Rodent Pokemon that look like they are part of some evolutionary line but they are separate from each other. The introduced Plusle which looks very similar to a Pichu oddly enough except it's ears are tipped Red and the Tail has a red Plus sign. It's counterpart is Minun who looks the same as Plusle only it has Blue Tipped Ears and a Blue Minus Sign for a Tail! They also make an appearence in the remakes of Omega Ruby and Alpha Sapphire!
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In Generation 4 the Sinnoh Games Diamond, Pearl, and Plantium the introduced a brand new Electric Rodent based off a squirrel and it's name is Pachirisu!
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In Generation 5 the Unova Games Black,White, Black 2, and White 2 they introduced a new Electric Rodent Pokemon based off a flying squirrel and her name is Emogla! Which marks Emolga the first Electric Rodent to have a second Type that being Flying making her complete Resist Ground attacks!
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Which brings up the last Electric Rodent Pokemon being Dedenne that was introduced. He is a unique one in which he is the only one with the brand new Fairy Type that was introduced in Generation 6 Kalos region games of X & Y. Along with him they created a new move which is better then Thunder Wave which only caused Paralysis. They created the move Nuzzle which only hits for 20 Special Attack but it's a guarantee to cause Paralysis as well. So why not take some hit points away as well with that move then use Thunderwave.
Electric Rodents are my love! *Squees*