The matter of this plural is something I happened to fall into by accident this week. I was working on this out of Cicero, De Natura Deorum 2.50:
Vomitione canes, purgatione autem alvos ibes Ægyptiæ curant.
Dogs treat their stomachs by vomiting, while the ibises of Egypt do it by purging the bowel.
I went to Wikipedia to see what the current plural of “ibis” was—I know, I know—but got no help there; some persistent vandals had taken it into their heads that it ought to be “ibi”.
I’ve run into worse cases than this before, but I’ll say it again: Just because it ends in something that sounds like -us—or even if it does end in -us—doesn’t mean it takes a plural in -i, or -ii, or anything like it. Of course the classical plural of “ibis” is its own special matter: it belongs to one of those sets of words that are attested being declined in a couple of different ways: with a stem in -i- and with a stem in -id-. (In a dictionary you’d see: ib|is, -is or -idis.)
Cicero, for example, uses ibes in the nominative plural twice and the d-less form (ibes or ibis, depending on your edition) in the accusative plural once, all in De Natura Deorum, as in the example above. (He also uses ibin for the accusative singular, though this is not a point for either side: the regular Greek declension would be ἶβιν whether the stem is in -id- or not.)
On the other hand, Ovid uses the form with -id- in his poetry—and actually wrote a whole vicious poem about a person he titled Ibis.
Anyway. The upshot of all this is that the Latinate plural of “ibis” in English could be either “ibides” or “ibes”. “Ibes” is, so far as I can tell, rarer, but neither of them is common; the purely English plurals “ibises” and “ibis” without an ending (like ‘grouse’) are more usual these days.