This is more of a "lore" question.
The basic issue is that races such as the Mahjarrat, Demons, and the Gorajo Stormbringers have wells of energy for them to tap into allowing them to cast without the need for runes.
Races such as Humans need runes because the majority of humans don't have that well of power in them. A perfect example is Kenneth from the Sea Slug quest series who was born with such a well of power. Similarly Safalaan is half human and half Icyene which similarly grants him some magical ability without the use of runes.
Normally, you would chalk up a creature in game carrying runes as a drop to the need from devs for something to add to a drop table.
Waterfiends for example
always
drop at least 1 water rune. Lesser/Greater Demons drop Fire Runes. A partial explanation could be that for creatures with that well of magic drop a rune that resonates with their Runescape (yes, this is a canonical thing ...) in their death throes. The energy focuses itself and a few runes drop down.
The other mundane explanations are that the runes were hold overs from adventurers that died to the creature or the creatures have them for some use in the event they get a little tired.
17-Jul-2022 14:41:42