This seems like a very obvious joke.
Feylines
Felis catus arcanus
Description: They’re domestic cats. Sure, yeah, Feylines do have an arcane blue-colored patch of fur on their brow. And, yes, they glow when doing a standard magical animal detection spell. Other than that, there’s not much on the outside to distinguish Feylines from other cats.
Abilities: Aside from the usual cat stuff, a Feyline will instinctively follow the path of any ley line it encounters. Unless the cat doesn’t feel like it, because it’s busy doing other cat stuff. But it won’t willfully ignore a ley line, either. Apparently Feylines find the sensation of being in a ley line to be pleasant.
The effect is easiest to observe when there are three or more Feylines present: one Feyline may be just wandering around in a straight line, two might have just happened to be going in the same direction, but three is where it really becomes obvious that Feylines can perceive magical energy conduits. Why precisely they can do this is a mystery of some interest to mages; Feylines have no inherent magical abilities themselves, and cannot use ley lines at all. Presumably the ability to do so was at first merely a favorable mutation that manifested in some cats after one too many exposures to raw magical energy.
But the reason why the mutation survives is obvious: mages find it extremely useful to have magical familiars who can see ley lines. They absolutely encouraged the mutation to breed true, to the point where Feylines are now a viable cat breed. Breeders have also encouraged traits in Feylines that make them better-suited for travel; some adventuring parties keep Feylines even when there are no handy ley lines to check out or investigate. Feylines make perfectly good mousers, companions, and remote viewing stations, after all.
Personality: They’re cats. Working cats, to the extent that any cat would be; but Feylines show a typical range of cat behaviors and characteristics. Some are friendly, some are aloof, some are ornery. But they all tolerate long trips and tight quarters a lot better than a regular domestic cat would.