In her case, it's a species called a "Raichu." Right now, she's a "Pikachu" and when she was born, she was a "Pichu," just to put that one on the record. Pichu are smaller than Pikachu and have cuter features and much smaller tails. Meanwhile, Raichu are larger, more powerful, and with much longer tails, which they apparently use to ground their electricity.