The Koturians were the native species of the planet Koturia.
Biology
The Koturians were a humanoid species with a wide variety of hair colours. They were capable of a form of regeneration called phasing, but unlike Time Lords, they can only do so once in their life, and only on their wedding day, and said wedding day must occur by a certain point in their life, after which it was no longer possible to undergo phasing. In order for the phasing to occur, a special stone was required as an energy conduit, and both parties must be truly in love. Usually, both the bride and groom undergone phasing, but it could still happen with one partner if the other was a non-Koturian. (PROSE: Something Borrowed)
Culture
Early Koturian explorers visited Earth in the late 20th century and were charmed by the glitz of Las Vegas, as well as its economic potential. They physically modelled their civilisation after Las Vegas, making Koturia another New Vegas, closer to the original if you discard the fact that Pterodactyls are much less common in Nevada.
Peri Brown stated that one of Koturia's hotel and casino complexes looked exactly like a hotel and casino in Vegas called the Sahara. Koturia was also famous for its weddings, but unlike its inspiration, it was considered "the height of romance", according to the Sixth Doctor. It was one of the wealthiest planets in the solar system and species came from far and wide to make their fortunes and get married. (PROSE: Something Borrowed)