Jon Miller the voice of the Giants has said that the term originates with the Giants―that is, the New York Giants, who used to play on the Polo Grounds in the late 1800s. According to Miller, there was a real bull pen out beyond the left-field fence, with real bulls in it! And the relief pitchers warmed up not too far from there.

The great etymological mystery of the game's diverse and expansive lingo.

No one knows the definitive origin of "bullpen". There are a few theories, some more plausible than others, but the true source has never been tracked down.

It is where bulls are penned up to later be released onto the baseball field

First reliever was named Bill en

it's catchy