The Sumxu is also known as the Chinese Lop-Eared Cat , Droop-eared cat, Drop-eared cat, or Hanging-Ear cat. All the names refer to its main feature - pendulous ears. Nowadays, the breed is considered extinct.
It is thought that the pendulous ears were a result of mutation similar to that occurred in the Scottish Fold.
All descriptions of the breed are based on reports of travellers. In 1976, a German naturalist gave rather a detailed description of the Sumxu, when a droop-eared cat had been brought from China by a traveller. The breed was described as long-haired cats with glossy black, yellow or cream coats and pendulous ears. Most probably, they looked like longhair Scottish Folds.
The Cat by Lady Cust (1870) has this brief description:
Bosman relates that in the province of Pe-chily, in China, there are cats with long hair and drooping ears, which are in great favour with the Chinese ladies; others say this is not a cat but an animal called 'Samxces'.
Georges Louis Leclerc, Comte de Buffon, described the cat in The Natural History of The Cat (Volume 4 of Histoire Naturelle c. 1767, translated by William Smellie, 1781):
Our domestic cats, though they differ in colour, form no distinct races. The climates of Spain and Syria have alone produced permanent varieties: to these may be added the climate of Pe-chi-ly in China, where the cats have long hair and pendulous ears, and are the favourites of the ladies. These domestic cats with pendulous ears, of which we have full descriptions, are still farther removed from the wild and primitive race, than those whose ears are erect.
Buffon added:
I formerly remarked, that, in China, there were cats with pendulous ears. This variety is not found any where else, and perhaps it is an animal of a different species; for travellers, when mentioning an animal called Sumxu, which is entirely domestic, say, that they can compare it to nothing but the cat, with which it has a great resemblance. Its colour is black or yellow, and its hair very bright and glittering. The Chinese put silver collars about the necks of these animals, and render them extremely familiar. As they are not common, they give a high price, both on account of their beauty, and because they destroy rats.
Jean Bungartz also described the Chinese Lop-Eared Cat or Hanging-Ear Cat in his book Die Hauskatze, ihre Rassen und Varietäten (Housecats, Their Races and Varieties) from Illustriertes Katzenbuch (An Illustrated Book of Cats) in Berlin in 1896:

The Chinese or Lop-Eared cat is most interesting, because it provides proof that by continual disuse of an organ, the organ withers. With the Chinese cat the hearing and ears have deteriorated. Michel says the Chinese, not only admire the cat in porcelain, but also value it for culinary reasons. The cats are regarded as special morsels and enjoyed particularly with noodles or with rice. This cat is bred particularly for the purpose of meat production, and is a preferred Chinese morsel; this is not unusual if one considers that the Chinese consume much the sight of which revolts the stomachs of Europeans. The poor creature is confined in small bamboo cages and fattened like a goose on plentiful portions. There is extensive trade with other parts of Asia and the canny Chinese allow no tomcats to be exported so there is no interference in this lucrative source of income.
Due to the restrictive conditions that have deprived the cat of its actual use, its hearing has decreased because it is no longer needed for hunting its own food. With no need for watchfulness, it had no need of sharp hearing to listen for hidden things so its hearing became blunted and as a natural consequence its ears lost their upright nature, gradually becoming lower and becoming the hanging ear that is now the characteristic feature of the Chinese cat. At first impression this is a surprising and amusing look, but this impression is lost with closer examination. If one ignores the characteristic of the ears, one sees a beauty similar to the Angora cat: a long, close coat of hair, albeit less rich, covers the body. The hair is silky-soft and shining and the colour is usually isabelline or a dirty white yellow, although some have the usual colouring of the common house-cat. In size it is considerably larger and stronger than a housecat. The ears hang completely, as with our hunting dogs and are large in relation to the cat.
Although the Chinese cat is found in considerable numbers in its homeland, it is rarely found at European animal markets. Only one such cat has reached us in the flesh; we acquired this years ago when a sailor returning from China brought it into Hamburg. The accompanying illustration is based on this cat. In character it is like the Angora cat and somewhat languid. It prefer to live by a warm fire, is rather sensitive to flattery, hears badly and is at its most animated when given milk or food. Apart from its unusual ears, it has no really attractive characteristics and is a curious specimen of housecat.
In Frances Simpson's The Book of the Cat (1903), contributing author H.C. Brooke wrote:
There is said to be a variety of Chinese cat which is remarkable for its pendent ears. We have never been able to ascertain anything definite with regard to this variety. Some years back a class was provided for them at a certain Continental cat show, and we went across in the hope of seeing, and if possible acquiring, some specimens; but alas the class was empty! We have seen a stuffed specimen in a Continental museum, which was a half long-haired cat, the ears being pendent down the sides of the head instead of erect; but do not attach much value to this.
In 1926, H.C. Brooke wrote in the magazine Cat Gossip that for many years Continental cat shows had offered prizes for the Drop-eared Chinese Cat. On each occasion, the cat failed to materialise and Brooke considered it to be mythical. Other writers suggested the folded or crumpled ears were the result of damage or haematomas. Brooke wrote that although no-one ever saw the cat itself, one always met “someone who knows someone whose friends has often seen them”. Brooke himself had been assured by a Chinese gentleman he had met only once that “he knew them well”.
HC Brooke, and several other cat fanciers, contacted the Chinese Embassy and Carl Hagenbeck's animal exchange in Hamburg and also a “certain well known author, who has lived for years in China and knows that country well”, but their enquiries bore no fruit. The search for this cat became so intense in the 1920s that The American Express Company instructed their representatives at Shanghai and Peking to make enquiries with the wild animal dealers who supplied zoos. They also had no success finding a Chinese Lop-Eared cat for western cat fanciers.
Brooke said that in 1882 he had seen a stuffed specimen in a Continental museum. The specimen was “half-coated with yellowish fur” and Brooke admitted it might have been a fake or a cat with its ears deformed by canker. With all avenues of enquiry finally exhausted, Brooke declared the Chinese Drop-eared cat extinct.
The last reported sighting of the Chinese Lop-eared cat was in 1938 when a droop-eared cat was imported from China. On that last occasion the mutation was believed to occur only in white longhaired cats.

Reference: Wikipedia