The antichrist is the personification of evil in the Christian world, variously the one who denies Jesus' divinity, and the one who tries to defeat him and make himself godly. A very bad seed indeed!
What does it look like? That has been left to the artists. This is a close-up of a fifteenth century painting of Saint Michael (see entire painting, below), who is making a name for himself by slaying the antichrist.
Note that this evil beast is made for the most part of animals. There is some kind of reptile or serpent coming out of the hair and a bird on the end of the tail. The arms are frog-like. Fish or eels are coming out the ears. All over the front are insect-like beings (they have eight legs, but the painter seems ignorant of the insect/spider thing).
The body itself is not divided into normal segments. It is a series of heads: The thorax is a head, with eyes and a ferocious mouth where the stomach would be. Horses' heads make up the thighs. The groin is another head, its mouth, with jagged teeth, where the vagina would be–surprise, surprise. Note its discreet beard.
Perhaps most interesting are the eyes. The normally-placed eyes are dim, sickly. But the eyes in the thorax and groin are brilliant, glowing. The heart sees, the gonads see, even the thighs see–but not the brain. Overall I count twenty eyes–not including the thousands on each of the insects/spiders.
This is what the painting says to the non-human: Your image of ultimate evil is a menagerie of animals, all of whom have roamed this planet far longer than you have (even the "mythological" ones closely resemble the real). The antichrist has a number of independent minds. And it has eyes everywhere.
In other words, it sees the world as it is, and makes of it what it chooses. No human nonsense, no constraints of any kind. No wonder it is Christianity's worst nighmare!
Here is the Magnificent Michael, all seven feet of him:
(Courtesy of Numbers the roach.)