A lot of good ones here. When I was a kid Hoot Evers of the Detroit Tigers was a scary adversary. In 1950 he almost helped Detroit win the pennant always 20HR's 100 RBI guy. The next year his average went down 100 points and was useless thereafter.
Karl Spooner came on to Majors like a lightning bolt at the end of the 54 season. Started & completed both starts, both shutouts. Without looking it up I think he struckout 27 batters in 18 innings and only gave up 6 hits in the 2 games. Hurt his arm and was out of baseball in 2 or 3 years.