Kestral (Rest and Re-Creation)

Kestral was a Zygon warlord who, with Anktra, planned to carry out a traditional duel on the planet Shontaa. This involved fighting to the death using a variety of host bodies. This duel was observed, however, by the Fourth Doctor and Leela, who were on Shontaa for a picnic. The Doctor, realising that the Zygons had the hosts they were mimicking locked up, found the Zygon ship and freed the prisoners.

Although Leela believed the Zygons would be killed, the Doctor pointed out that they were the only ones who could fly the ship. (DWY: Rest and Re-Creation)