OK, I guess I wasn't thinking of RQ monsters as "races" in the relevant sense - because in 3E a gnoll, say, or a wyvern, even before you get to treat it as a "race" to which you can add class levels, has a "monster level" which dictate its mechanical features.The PC build rules are pretty much how people make NPCs for Runequest. Take a race, add a background, add a prior career, add any other relevant factors
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So if you want a wyvern who is a priest of the Sun Dragon, take a wyvern, add the priest career, stick the cult magic in there, and you're mostly done.
So in 3E there is a mechanical connection between (say) the gnoll's hit points (via its hit dice) and its skill bonuses and its feats, whereas in RQ those two things are distinct - you can give a monster/race, like a wyvern, a certain stat spread, and also a default skill spread, without having to jump through the 3E-style hoops of explaining how one is related to the other.