The Yehat are fearsome and honorable pterodactyl-like creatures who were allied with the Alliance of Free Stars in Star Control. For some reason they speak with a notable Scottish accent when their language is translated to English. The Yehat are ruled over by a Queen, who rules over the various Yehat clans. Historically, this has been one from the Veep-Neep Clan. The Yehat are very close allies with the Shofixti.
In Star Control I the Yehat are the backbone of the Alliance Starfleet, due to their advanced weapons technology and their intensely martial society. The rule of the Veep-Neep Queen is based on the Yehat never being defeated under a Veep-Neep, so to save face, the Queen orders them to defect to the Ur-Quan side late in the war. This is a source of great shame to many Yehat, compounded when their friends the Shofixti chose to annihilate their species in a blaze of glory rather than surrender.
As such, at the beginning of Star Control II, the Yehat are Ur-Quan battle thralls, and while sympathetic to the human Captain they will not give any aid. Once the Captain brings them evidence that their adopted children, the Shofixti, are still alive, it triggers a civil war among the Yehat. Eventually the rebels win and the Veep-Neep Queen is deposed and the Yehat join the New Alliance.
The Pkunk are an offshoot of the Yehat race. In Star Control II they decide to reunite with their Yehat brethren, despite the fact that the Yehat despise them. The human Captain tries to dissuade them, but eventually the Pkunk make contact with the Yehat and the Yehat seemingly destroy them. In the final battle against the Sa-Matra, a combined Pkunk and Yehat fleet arrives to reinforce the Captain, and they reveal that after much perseverance the Pkunk persuaded the Yehat to accept them after all, and the Yehat even chose a Pkunk Queen as the replacement for their deposed Veep-Neep queen.