Iris, Iridis (noun)
– declension: 3rd declension
– gender: feminine
1. Iris (messenger of the gods, goddess of the rainbow)
You can identify third declension nouns by their genitive singular ending ‘-is’. See nationalarchives.gov.uk and Wikipedia.
In contrast with the first- and second-declension endings, those of the third declension lack a theme vowel (a or o/u in the first and second declensions) and so are called athematic.
Note via Toby O.