Nekomata (猫又, "Forked cat") is the name of a fictitious being from Japanese mythology. It is said to be a lower form of a Bakeneko (a high ranged cat-demon). Japanese folklore says that Nekomata looks like a domestic cat, but they have twin tails instead of just one.
As it said in the anime most people resent the Nekomata because they thought they were evil. Kirara and Kuroro are two examples of nekomata.