Q. Should the common name of a species from a non-English language be treated as a foreign word and italicized, or should it be left in roman type? I’m thinking of the bird known as a po‘ouli in Hawaii, which is elsewhere called the black-faced honeycreeper. Should po‘ouli be italicized?
A. Though it’s not listed in Merriam-Webster (as of July 5, 2022), the name po‘ouli seems to be relatively well established in recent English-language publications that discuss that bird (sadly reported extinct in 2021); in fact, a Google search for “black-faced honeycreeper” brings up “po‘ouli” first, suggesting it’s more common now than the common English name. So you shouldn’t need italics to refer to a po‘ouli except when using the name as a word (as in the first sentence above and the last sentence in your question).
But if you were to refer to, for example, a Deutscher Schäferhund—the German name for a German shepherd—italics would help signal that the German name would not normally be used in an English-language context (except, for example, to let readers know what that name is).
In sum, sometimes it’s necessary to go beyond the dictionary as a rough gauge of a term’s familiarity in English contexts. For the glottal stop (or ‘okina) in po‘ouli, see CMOS 11.70 (under “Hawaiian”). For advice on capitalizing dog breeds, see this Q&A.