Demons can be found in almost every culture or religious group. Typically described as a type of spiritual or inter-dimensional being, they could be used to explain a vast number of cryptids, including ghosts, ghouls, aliens, etc. Most cultures associate Demons with beings who have dark spiritual powers. They have been blamed for possessing people, haunting places, causing sickness and sometimes death. Just as common as demons in folklore are the exact opposites, which are typically messengers from a god or gods. It should be noted that not in every culture were demons solely bad or evil spirits, in the Greek culture, before attacking a city, they would pray to that cities gods asking them to change sides, if they won the battle, they would build a temple and make those gods a smaller god in their mythology. after a time, the greeks would blame bad things on these smaller gods, and they would call them Demi-Gods, but the Demi-Gods where not seen as evil.
In most cultures with demons, some people were considered to have made deals with these Evil creatures. These people sometimes gained wealth, or perhaps supernatural powers. Sometimes these people were put in positions of high power within the culture, and sometimes they were hunted down and killed for being witches.
When most refer to a Demon in a literal sense, they are speaking of the spiritual being of evil from the bible
Some explanations include; inter dimensional beings, ghosts, ghouls, or horrible people doing evil things and wanting to blame someone or something else.
Possibly underground reptilian species.