Well, strictly speaking, Andreas, they could populate empty spell lists from scratch, so it wasn't so much spell invention as it was stitching together pre-existing spells to form new groups. The class was called an an Arcanist - it was actually in Rolemaster Companion II methinks (maybe III). What they did present in the first Companion was 'Arcane' magic, a new uber-realm of magic which allowed its practitioners to learn lists from any realm (called an Archmage). At first, Arcane magic was an escape route from the division of magic into discrete realms. Later versions of the game changed Arcane magic into 'just another realm', with it's own professions and their base lists, etc. The Arcanist, then, was a spell-user of the arcane realm that could build lists of spells from all of the pre-existing spells.
Ok. Godsdamn* this spell system! I love it - but whew! Too much! Anyway, non spell-using characters had to choose a realm of magic, even the grunts, sages and swashbucklers. This decision could have some in-game consequences for them, but was largely superfluous. Spell-using classes had their realm (or realms) pre-determined. Here was the first clash - the Monk. By-the-book, a Monk is a semi-spell user of the Essence realm ('essence' is a term analagous to 'ambient magic', 'the force', it's practitioners those who learned formulae and methods to coerce specific effects into being). Early on, a player wanted to make a Monk, and saw them as belonging squarely in the Mentalism camp (mentalism: remember the magical field of the Essence? Yes. Well, it is uneven. Living beings form nodes in this field, and Mentalist casters tap this personal allotment of magic). I agreed. These realm divisions are semantic, or at best/worst, settling elements - and so in my game, the Monks were Mentalists.