Essa é uma boa pergunta. A própria JKR não parece saber:
JKR: And did you like the dedication? "You were right all along about 'Horcri'?"
SU: Oh, no!
MA: Oh, Jo!
SU: There's that word!
JN: Nobody would believe me.
MA: Oh, he's never gonna let us--
JKR: So, I did write him a letter. And I did say that, actually, "horcri" is-- I mean, it occurred to me, you know, it really would've been, I think, a more correct plural, but I already had "inferi", and I didn't wanna have too many, umm, you know, Dark Arts weapons ending in "R I", so it was a kind of stylistic choice, really. I like "horcruxes" as a plural. - Pottercast Interview : Part 1, 2007-12-17
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JN: Until next time, guys, keep twiddling those dials.
JKR: The next password will be: "Horcri". Pottercast Interview : Part 2, 2007-12-24
No entanto, em seu antigo website ela declarou explicitamente que era Horcrux es . Esta página foi arquivada pela primeira vez em 2006-06-12 e arquivada pela última vez em 2011-08-06.
Q. Is the plural of 'Horcrux' 'Horcri?'
A. No, the plural of 'Horcrux' is 'Horcruxes', as demonstrated by the eponymous chapter in 'Half-Blood Prince.'