My story is essentially about both Marble Pie and Fluttershy falling in love with Big Macintosh (and a dozen other mares just falling in love with his cock), and deciding that he has a big enough heart to love both of them. But neither really love each other as much as they love Mac.

It's not harem since there is a clear distinction between how he loves the two of them and they love his personality and soul, and others who just are addicted to how he makes them cum. So is it still polyamory?

5035520
Polyamory relationships can indeed be focused around one individual. The difference between an open relationship and a polyamorous one, is that an open relationship involves two people dating and being okay with one or both of them having sex with others, while a poly relationship involves commitment between more than two people, but not neccisarily that everyone is committed to each other.

Granted, it sounds like this is both an open and a poly relationship, but that's neither here nor there.

Going by the wikipedia. I'd think it would qualify as:

"So-called "geometric" arrangements...Examples include "triads" and "quads", along with "V" (or "Vee")"

So, yes?

5035532 open more out of necessity. Marble and Fluttershy together can't take the pounding Mac gives out as often as he needs it, and the others can't deal with being denied it