Possibly inherited from Late Latin cattus (“cat”), having later been suffixed with -oi (or perhaps directly from a Vulgar Latin cattōneus), the original pronunciation thus being *cătoi. The change from ă to o is not uncommon, occuring in words such as; porumb < părumb and boteza < *băteza. See also cătușă, which meant cat in archaic language. Alternatively from a former *cot, from Old Church Slavonic котъ (kotŭ), from Proto-Slavic *kotъ, later being suffixed. Though it's possible the pronunciation is simply a result of Slavicisation.