Chronomancers (which is an Onirian term for them, also known as the Lost Peoples (often derogatively, by Umichooans) are a race of nonhuman beings that come from an entirely different universe that “is not there anymore”, tend to take human forms to interact with the locals in our reality, and seem to have a control of the flow of time way beyond what any local civilization can manage.
The first Chronomancers were spotted some centuries ago in the Onirian Time Branch, but given both the nature of the Veil and their skill at passing by regular humans, they might have been around for a lot more time.
Assuming they are just not time-traveling to the centuries they like, because apparently they can do that.
The original Chronomancers came from a time stream that resulted from an entirely different Big Bang than the one that spawned ours, and in their home reality, the laws of physics, and particularly the way time worked, were subtly different. This means that the shapes they originally had were totally incompatible with spacetime in our corner of reality.
Instead, the first exiles that reached our world put themselves into shapes that actually fitted into it, and made some of their servants into humanoid shapes so they could move around the versions of Earth easily (and presumably other shapes fitting entirely different dominant species in entirely different planets and time branches).
According to Mariette Navia, the lesser members of their civilization are built to specifications to fulfill specific purposes, but she admits that she has both significant memory issues and a huge hatred for the Chronomancers in charge.