The auxiliary medical probe (abbreviated AMP) were robotic mobile medical units that were used by the Vostigye Union.
In an alternate timeline where the USS Voyager was forced to join the Vostigye Union, The Doctor's program was uploaded into the Vostigye computer system and given control of all the AMPs. This allowed the Doctor to provide care on all Vostigye vessels and settlements, while keeping his personality.
The Doctor sent an AMP in a small unmanned craft through Swarm space to Vidiian space to help cure the Phage. The Vidiians found the AMP and captured it to learn the location of the Voyager, forcing him to wipe the probe's memory. But by sending it again, they finally allowed him to help. (VOY - Myriad Universes novel: Places of Exile)