Why do Siamese cats have a kink at the end of their tails?

It is difficult to say. Early Siamese cats that were imported from Thailand had a kink in their tails, and for many decades this trait, along with Siamese cats being cross-eyed, were seen as proof that a cat was indeed of pure Siamese breeding - especially in cat shows.

Today, this trait is now considered a flaw within the Siamese breed and has largely been bred out, although it still persists among street cats in Thailand. Siamese cats bought from reputable breeders should not have kinks in their tails due to careful breeding programs.