Thisby, Unknown Time
Puck and Sean's home, Thisby, is a small island off the coast of… well, we're not sure what it's off the coast of. References are made to a "mainland," but we're not sure which mainland that is either.
The myth of the water horse originated in Celtic mythology, which would place Thisby in Ireland. However, none of the characters speak with any sort of dialect, so it's still difficult to pinpoint their location.
Although Thisby has beautiful craggy cliffs, it smells like rotten meat sometimes and the tourist industry is basically non-existent until the Scorpio Races roll around every year. Tourists must prefer bloodshed to scenic vistas.
Thisby is like those small towns in which everyone knows everyone and all their business. Puck and Sean don't mind it, because they care more about horses, but to Puck's older brother Gabe, it's borderline claustrophobic. He describes Thisby as, "This island [...] That house you and Finn are in. People talking. The fish—goddamn fish, I'll smell like them for the rest of my life. The horses. Everything. I can't do it anymore" (38.69). But Gabe—Skarmouth has a butcher shop and a hotel. It's totally cosmopolitan.
What sets Thisby apart from most boring islands is that it's the only island with the water horses. There's even a cave painting of one, suggesting that water horses have been there for quite some time.