Picking the Ancestral Mech background option will trigger an event a day or two into your career that adds a light 'mech to your mechbay. The exact 'mech in question differs based on which option you picked in the "Before Becoming a mercenary, you came from..." field. The following is the list of the 'mechs provided with each origin.
The Rimward Periphery: Hornet HNT-AM
The Deep Periphery: Stiletto STL-AM