But not a geometrical perfect triangle.

Depends on your chosen space. Spherical geometry would work no?

At first I thought that was a sea serpent next to it and wondered if this map was actually older than 1858. Or do they still exist?

It doesn't transmit well through text unfortunately. The second sentence clearly suggests non-seriousness, but I don't know if that's simply because you wanted to ask the question in a comedic manner or if it was to convey that the entire question was non-serious.

Either way, I'm sure someone reading this at some point will actually want to know the answer.

It’s not that kind of party luv.

Hah, it's the "S" of "BARNSTABLE COUNTY", but I've got no proof that it's not positioned to hide a leviathan.

Oh, for sure. Þe oþer similar period map is less stylized, þough, and it also shows a triangle.

Perhaps surveys weren't accurate and naming followed perception, but it appears þe lake has been changing over time and I believe it's entirely possible it was more triangular when it was named.

Which is odd, because the second map looks to be from at least 1890, and therefore newer than my map!

It's probably just shrinking. In a few years it'll be triangle puddle.

Objectively worse. Get your shit together triangle pond.

What are all those Percival and James type names for?

Just rename to Triangular Pond and call it a day.

I'd guess it's the owners of homesteads / houses.

This pond would probably fit pretty nice on the right hyperbola

True. A big enough triangle (or a small enough Earth) would show a curve. But I think it's convex, not concave.

Cartographers be lazy?