The Whaleriders are mentioned when April is catching up with Brian at the Journeyman Inn, and asks him where he plans to go next. He tells her: "it's off to sea again. I've been thinking about heading east, perhaps convincing a whalerider to take me along on a hunt."
The name 'Whaleriders' may indicate either an entire people or simply an occupation. Given Brian's description, they probably dwell on an island in the Great Ocean east of Marcuria, perhaps in or near the Bristan Atoll.
This concept was most probably inspired by Whale Rider (1987), a novel made famous by its 2002 film adaptation, about the daughter and sole-surviving child of a Maori tribal chief who defies gender roles to ride a whale in recreation of the tribe's founder, proving her claim to leadership.
The Maori connection may stretch to Herman Melville's Moby-Dick (1851), which would certainly be in keeping with Brian's depiction as a Hemingway-esque adventurer.