From Abracadabra to Zombies
In a nutshell: Monsters are freaks of nature or scary creatures in stories that are ugly and mean.
The word 'monster' comes from the Latin monstrum, which means something like a freak of nature. So, technically, something like a two-headed snake would be a monster. Two-headed snakes happen when there is an incomplete splitting of the egg.
Animals are made up of cells, which are like chemical factories. One of the most important strings of chemicals in cells is the one that copies itself to make more cells. These chemical strings are called chromosomes. Chromosomes come in pairs, one from the mother and one from the father. Each chromosome has several genes. The genes help make different kinds of cells in an animal's body. Sometimes when genes are copied the chemicals get mixed up. Sometimes the chemicals get mixed up when an egg splits to make twins. That's what happens when a two-headed snake is made.
The science of genetics tells us that the bodies of animals grow the way they do because of genes. The genes in most human embryos will produce people with various amounts of hair. Some people, though, because of a problem gene will grow up to have an unusual amount of hair all over their bodies, a condition known as hypertrichosis.
We might call those without hypertrichosis "normal" and those with this disorder "abnormal" or freaks. But both are what they are because of their genes. Something unusual happens once in a while when those genes are being copied. We call these unusual changes in genes mutations. Some mutations are not good and they lead to things that don't help us survive or have children. Sometimes many mutations over many generations can be very good. Something without feathers might have the first mutation that combines with other mutations in various descendants over millions of years. Eventually some of those descendants might evolve into something with feathers. Something with feathers might have the first mutation that combines with other mutations in various descendants over millions of years. Eventually some of those descendants might evolve into something with wings. Something that can't fly might have the first mutation that combines with other mutations in various descendants over millions of years. Eventually some of those descendants might evolve into something that can fly.
Natural monsters like two-headed snakes and people with hypertrichosis are interesting, but most people think of scary creatures in stories that are ugly and mean when they think of monsters. Monsters appear in stories to terrify and challenge others in the story. Odysseus on his way home from the Trojan War has to fight the Cyclops, a one-eyed giant. Many heroes in stories have had to go up against fire-breathing dragons or other hideous creatures.
Mary Shelley (1797-1851) in her novel Frankenstein: or, The Modern Prometheus (1818) described the monster created by Dr. Frankenstein as eight feet tall and hideously ugly. His skin was yellow and tight over his body so that it "barely disguised the workings of the vessels and muscles underneath." He had glowing eyes and black lips. The only person not afraid of the monster in her story is a blind man. In movies, the monster is usually called Frankenstein and is shown as ugly and deformed.
It is true that some monsters are shown as gentle and kind, even if not very good looking. Most of the time, though, monsters are created to scare people, either people in the story or those reading, listening to, or watching the story.
Some popular modern monsters—especially in films—include aliens, Bigfoot, the chupacabra, Mothman, vampires, werewolves, and zombies. Some strange lake creatures are called monsters, but they're not very scary or ugly. On the other hand, I wouldn't want to be swimming in a river with a hungry crocodile or school of piranha!
Many seafaring men have reported seeing monsters while at sea. Stories of sea serpents, sea dragons, and giant sea creatures of all sorts are a part of cultures that have contact with the sea. Some of these monsters might be whales or giant squid. Sea monsters aren't known for their ugliness so much as for their size. On the other hand, when large sea creatures decay in the ocean, their remains can look hideous when they wash up on shore, giving rise to more stories of monsters from the deep.
For an awesome photo of a sea monster see 55 FOOT SEA MONSTER WASHES UP IN CHINA .
What is a gene? from KidsHealth.org