Sure looks like the ferret I had, about the same size and color, my ferret just had a raccoon mask, but colored much like that one.
mine may have had a shorter tail than in the pic so maybe it is a weasel..... tough to stupid cat I guess.
The ferrets I had one was all black and the other was brown with white belly and a mask so I guess they come in all colors.If it is a weasel try and find their home and do em in---If you have any young kids around and they find them they will be in a world of hurt plus rabies shots.
They are for sure tought buggers and fearless,they don't back down and fight to the death.
You got one Bad A** cat ----Give em a special treat.