Permanent magnet DC motor earth wire
If it is an earth wire then it's probably there because the motor windings do not offer full reinforced insulation. This might be because the motor was (or is) intended to be used with a 12V power supply that isn't necessarily isolated from AC mains voltages and, the earth wire will protect users from receiving electric shocks should they touch the motor. In effect if a fault develops potential live voltages will cause an earth current and either a fuse or breaker will trip rendering the installation safe.

It could also be that the earth wire is for EMC reasons but without a clear data sheet or comment from the supplier, play safe; use a meter to determine that the power wires are insulated from the "earth" wire - if there appears to be some form of galvanic connection then you really have to get information back from the manufacturer.