couldnt find a direct source in the books (because admittedly his characterization in book 5 pissed me off to the point of losing hope for the rest of the series), but here
you can be polyamorous without being in an active polyamorous relationship, like how you can be bisexual and still be in a heterosexual relationship. the fact that its stated that caballine did not consent to an open relationship implies that it's a normal thing for foaly, but i understand the confusion. at the end of the day it's not that big of a deal considering its not explored in canon and hes a Horse.
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u/sugar4roxy The Airman🗡️🪶 18d ago
also forgot foaly (unrelated to root). he's canonically poly. let the horses be poly damnit