"I see one cat."
Translation:wa' vIghro' vIlegh.
For the same reason we have Denebian slime devils, Cardassian voles, Ceti eels, Regulan bloodworms, Bajoran bats, Ligorian mastadons, and a whole host of other "alien" creatures. We apply an English word to an alien creature that resembles its namesake. So Klingons apply a Klingon word to an alien (Terran) animal that resembles its namesake.