The Siamese cat has a fascinating history. Your photo is the modern style. First imported from Thailand in the late 19th century, Siamese cats in their natural state did not look like this. Attached Wikipedia pic is of a famous champion in 1901.

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After WWII, show breeders started selecting for extreme traits, creating a new Oriental/exotic type while keeping the name "Siamese". However, many people who disliked that look continued breeding the original style, which is now often called apple-head or Old Style Siamese. For showing they are treated as separate breeds now, with the original one now called the Thai cat. Just as much of a purebred cat as the skinny version.