The serpent in Eden was not a supernatural being (often called the devil, or Satan), it was an animal. Genesis 3:1 says the following about the serpent:
…the serpent was more crafty than any other beast of the field that the LORD God had made….
Here the serpent is identified as simply one of the beasts of the field.1 Genesis 2:19 tells us about where these animals came from: God made them from the dust:
Now out of the ground the LORD God had formed every beast of the field and every bird of the heavens and brought them to the man to see what he would call them. And whatever the man called every living creature, that was its name.
The serpent wasn’t a supernatural being, or a supernatural being disguised as an animal; it was simply one of the animals that God had created from the ground.
1. For passages with similar Hebrew sentence structures as Gen. 3:1, see: Gen. 3:1; 34:19; 37:3; Ex. 18:11; 19:5; Num. 12:3; Deut. 7:6-7; 7:14; 1 Sam. 9:2.