The Detroit mystery cat that is frightening the people living in one of its suburbs is either a serval (there are some serval characteristics) or a first generation serval hybrid: a Savannah cat. The mystery cat’s description neither fits the serval nor the first generation serval hybrid, the Savannah. It somehow falls between the two. A Savannah cat is a serval to domestic cat cross. One last option is that the mystery cat is a serval to wild cat hybrid (wild cat x wild cat).
The body conformation is a good match for the lanky serval and Savannah. There is banding around the legs which is not a serval characteristic. The head is small which certainly is a good match to these cats.
The tail of the mystery cat is too long for the relatively short tailed serval. The tail matches more accurately the tail of a Savannah cat.
However, the Detroit mystery cat has been described as being 4 feet tall. That is likely to be an exaggeration. A person who was close to the cat indicated it was about 2 feet to the shoulder or a little taller. Although that is a guess. If the cat is 2 feet and more at the shoulder it is very unlikely to be a Savannah cat as the world’s tallest Savannah is Magic at 17.1 inches to the shoulder.
However, a local person may be creating a different sort of serval hybrid. As mentioned, it may be a wild cat species to serval cross. I have not heard of such a cat. The pictures and video on this page make good comparisons, I think.
This is the Detroit mystery cat (picture taken by a Detroit resident using a mobile phone):
This is a first generation Savannah cat (Magic) – this photo is copyright Kathrin Stucki:
This is a female serval (the video was made by Michael at PoC)