Cowgirl

A cowgirl is someone — conventionally a woman dating a polyamorous person — who enters the relationship not to be polyamorous but to extract their partner from polyamory and 'rope' them into a monogamous couple, cutting the other partners out. It's the gender-flip of the established term cowboy, and the herding metaphor is the same: separating one person from the herd to keep for yourself.

Cowgirling can be deliberate or semi-conscious. Sometimes it's a stated plan; more often it shows up as steady pressure — subtly competing with metamours, framing the other relationships as obstacles, angling for rules that crowd them out, treating monogamy as the natural 'winning' endpoint. The damage is to the whole polycule, since it positions a partner's other relationships as rivals to be defeated rather than people to coexist with.

Naming it isn't an accusation against anyone who turns out to prefer monogamy — that's a legitimate discovery. The cowgirl pattern specifically describes entering a polyamorous relationship while quietly working to dismantle its polyamory, rather than being honest that monogamy is what you actually want.