Alright, so many demons seem to have formerly been angels. We all know the monstrous Fallen and others along with the less obvious phantom guards with their vestigial wings. Even Samael resembles Darksiders: The Abomination Vault's description of a Lost Angel. So, were demons created by the Creator from nothing or made from angels? Or was the Dark One, the as yet unrevealed former ruler of Hell, the first demon that twisted the rest? Silitha and some of the rest seem to indicate that demons have children, so maybe they become less and less angelic through the generations? Or can other beings be twisted into demons?
Silithia and the Chosen are not demons but creatures from the Abyss as stated by Uriel explaining their non humanoid form.(toxin1241)