While most are familiar with the specific rules regarding the care of Mogwai, the alien species introduced in Gremlins (1984), they also have a unique biology, physiology, and history which is integral to the greater understanding of their origin.
In Gremlins, Randall Peltzer stumbles upon Mr. Wing's curiosities shop and is introduced to the Mogwai. Though Wing refuses the Peltzer's request to sell, Wing's grandson sells him one in secret, then delivers an ominous warning involving the creature's care. The rules are just as iconic as the creatures themselves: don't get them wet, don't expose them to bright lights or sunlight, and never feed them after midnight. Peltzer's intention is to bring the Mogwai, who he named Gizmo, home as a gift for his son, Billy.
During the film, these rules get broken, which causes the Mogwai to reproduce into scaly, almost reptilian looking creatures called Gremlins who are much more deadly than their fluffier counterparts. While the Gremlins eventually are taken care of and Gizmo remains his cute, cuddly self, the notion that they are so fragile makes them a rather unstable and unpredictable choice of pet. At the end of the film, Mr. Wing comes to retrieve Gizmo, stating that humans aren't ready for such a "gift."
The Mogwai's Strange Biology
In their natural form, Mogwai are small, furry creatures with big eyes and large ears that are similar in appearance to the bush baby. Unlike marsupials and mammals with whom they bear a resemblance, Mogwai have no tail and walk on two legs. Their ears appear to be made of soft, thin skin with obviously present vasculature like a bat.
As stated in the rules regarding their care, they should not be exposed to sunlight or bright lights of any kind because this will kill them. Gizmo, in the film, shrieks "bright light" whenever this exposure happens, almost as if the light sensitivity causes him physical pain and discomfort. When exposed to water, they asexually reproduce by shedding balls of fur. The number of Mogwai that are produced is unpredictable. The creatures, on the whole, are innocent and disinterested in harming humans, but their much more dangerous counterpart is far from harmless. Feeding a Mogwai after midnight triggers a strange biological reaction, transforming them into scaly, clawed creatures with reptilian traits, though they retain some tufts of hair and bat-like ears. These iterations of the creature are different, geared toward mischief, destruction, and other forms of evil.
Where Did Mogwai Come From?
George Gipe, who penned the novelization, created the Mogwai's origin story and history beyond the original film and its sequel, Gremlins 2: The New Batch. The origin of the word "mogwai" is Cantonese and means "demon" or "evil spirit".
According to Gipe's lore, Mogwai traditionally reproduce during the rainy seasons which is likely why they, biologically, multiply when they get wet. According to the novelization, the original Mogwai can live to be thousands of years old and don't have a traditional language because their brains are capable of processing thoughts faster than words can form. Any created Mogwai have shorter lifespans than their parent and don't possess memory of their reproductive process or transformative abilities.
The Mogwai were created by a scientist named Mogturmen. Mogturmen wished to create a species that was highly intelligent, gentle, capable of reproducing quickly, and were able to live under a variety of different conditions. Due to resilience, they needed a benevolent nature and a high capacity for morality. This explains how Mogwai cannot kill their own kind. During Mogturmen's genetic engineering with the Mogwai, it was the intention to distribute them to neighboring planets as galactic diplomats who could restore peace to every society. When Mr. Wing says that the Western world isn't ready for Mogwai, it implies that humans aren't responsible enough nor ready for peace. Eventually, Mogturmen stopped the experiments on Mogwai when it was discovered that, due to their Gremlin side, few retained a gentle nature. The species on the whole was unstable, but Mogwai had already been dispatched to several planets, including Earth.