While the Arkranians were the first demons, they were not the first race of Demons to appear. The first two races to appear were the Xarkrains and the Arkranians. Every other race is a descendant of those two origins, but all demons originated from dragons. A chart of the origins and descndants is below.
In-Depth with Races
All the races eventually lost their dragon-like appearance (except the Altherans), but their ancestry is still the same. The reason why the Xarkrains and Altherans were not classified as Arkranians is mainly because of their appearance. The Altherans were too monstrous to even be classified as demons, and the arkrains were too dark and powerful to be considered Arkranians.