The Mystery Cow was a part of Phoney Bone's scam at the Great Cow Race during the events of The Great Cow Race. The Mystery Cow was not an actual cow, but the poorly crafted costume of Smiley Bone, who wore the costume in the race. Typical of Phoney's usual efforts in the series, his plan was foiled by Lucius Down.
It is not known what happened to the costume after the race.