From TARDIS Index File, the free Doctor Who reference.

The Millennium Bug was a name jokingly given by Alex Hopkins to an alien he sent Jack Harkness after on New Year's Eve 1999. It had 18 legs stacked with poison. Hopkins sent Harkness after the creature in order to distract him from his plans to kill the rest of the Torchwood Three team; he accomplishes this task before Jack returns, and then commits suicide himself, leaving Harkness in charge of the unit. (TW: Fragments)