There were some stats for generic demons in Wyrms Footnotes, including the Sembraki which were demons with fiery whips, not Balrogs at all.
Demons are normally malign supernatural beings. They differ from things like ghosts and elementals in that they have to be summoned and are normally quite powerful.
Among trolls, Demons include Dehori, Ice Demons, Wraiths and even Dara Happan Angels, who are very malign to trolls. So, demons can be culturally specific. But, summon a chaos demon and most people will recognise it as being a demon.
For non-Gloranthan games, things become more and less complicated (ho, ho).
They are more complicated because there are many more worlds and many different cultures, so how can you define what a demon is in each and every culture on each and every world?
They are less complicated because most worlds/settings have fixed ideas about what demons are, how they are summoned and what powers they have. So, summoning Stormbringer Demons is different to summoning Judeo-Christian Demons in medieval Europe.
Some demons when summoned act as deities and need to be worshipped, others give powers to their sumoner and then return to the Abyss, others stay around and rip people apart. It depends on the type of demon, the game setting and what the GM or player wants from the demon/storyline.