Per the Ecology article in Dragon, they (aside from the Marut) were created by a now vanished, militantly expansionist LN race known as the Aphanct.
Some folks online have speculated that the Aphanct had some relationship to either the extinct LN/LE natives of Acheron known as the Hassitor, or as remnants of the LN race that Asmodeus and the baatezu were part of prior to their fall to LE.
Also, back to canon, another planar golem, the blade golem, was created by the bladelings of Acheron.
FWIW, in Pathfinder the Inevitables were created by the LN Axiomites as construct armies for their wars against the Proteans of the Maelstrom.