The Baseball bat is a cut weapon from Zombie Master, it was an very early weapon in the Alpha Version, but the creators of the mod scrapped it because 'It was stupid and dumb for a HL2 mod', he was then replaced by the Imp. Crowbar.
However, together with another scrapped weapons, it can be playable on custom servers.
Useless baseball bats can be found in some maps.
In the first gameplay of ZM, a player can be seen attacking a Hulk with a Baseball Bat.