Yes, there is such a thing as a chocolate cavalier! This is a colour that was disallowed for showing in the 1970s (along with all black cavaliers and black and white cavaliers). These alternative three colours are known historically going back hundreds of years, for the small spaniels that are the ancestors of cavaliers. But only four colours are now officially recognised in the breed: blenheim (red and white), ruby, tricolour and black and tan.
Chocolate cavaliers, though rare, appear now and then due to a recessive gene for this colour that exists in some lines. Because the gene is recessive, it is impossible to remove it entirely from the breed, and so it continues to pop up now and then. There's nothing "wrong" with a chocolate cavalier; it would just not be considered an official colour and the colour's rarity makes them a bit of a novelty for anyone who owns one!
Unfortunately a website link that did carry pictures and a genetic discussion of cholcolate cavaliers no longer has those pages available.