Plural of “Animus/Anima”

Solution 1:

According to the OED, the Jungian terms anima and animus have no plural form in English.

animus /ˈænɪməs/.

No pl.

Etymology: a. L. animus (1) soul, (2) mind, (3) mental impulse, disposition, passion.

1. Actuating feeling, disposition in a particular direction, animating spirit or temper, usually of a hostile character; hence, animosity.

2. Psychol. Jung’s term for the masculine component of a female personality. Cf. anima.

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anima /ˈænɪmə/.

Psychol.

Etymology: L., ‘mind, soul’.

Jung’s term for the inner part of the personality or character, as opposed to the persona or outer part; also, the feminine component of a male personality. Cf. animus ².

They are in essence mass nouns, not count nouns.

In English, that is.

In Latin, where they do not mean what they do per Jung, anima and animus are unremarkable nominative singulars of the first and second declensions, respectively. They therefore form their nominative plural and genitive singular as anima > animae and animus > animi, and genitive plurals as anima > animarum and animus > animorum. So soul, souls/soul’s, and souls’.

• Nullum autem est, nisi quod animus ex se sibi invenit.   ― Seneca the Younger
  [~ “There can be no good except for what the soul discovers for itself within itself.”]

• Magnificat anima mea dominum.   ― J.S. Bach ☺
  [traditionally translated as “My soul doth magnify the Lord.”]

Solution 2:

Merriam-Webster gives plurals when they form irregularly, as in louse, child, or sheep. M-W doesn't comment at all when a noun has no plural, like mathematics or information. In the case of animus/anima, M-W doesn't list a plural, which to my mind indicates the M-W considers it either (a) without plural; or (b) with a plural formed along standard lines.

In English, often a Latin ending is used for one meaning, and the anglicized ending is used in another related, but different way. Index is a good example. In technical writing, index can be either a measurement or a table of references at the back of a book. The former is pluralized indices and the latter is pluralized indexes (anglicized). This precise usage is common, but not entirely standardized, I should think.

All this is to say that if you find yourself in a situation that requires a plural for animus or anima, you might use something of the same approach: -es/-s for general, and -i/-ae for technical (like Jung's use), or vice versa.