foramen magnum"

It's a Latin phrase. In Latin, adjectives usually come after the nouns that they modify. The phrase is used in English (in a technical anatomical sense only) in the Latin form. It is not modified to match English grammar rules.

Latin is still common in the biological sciences, medicine, law, and in the Catholic Church.
There are still remnants in everyday language such as etcetera.

Of course, very many regular English words also have Latin roots. Most have been slightly altered or adapted, but some are unchanged from the original Latin (e.g. "stadium" or "abacus", to give two totally random examples).

GPYsome are unchanged from the original Latin

And some even retain their Latin plurals.
radius; radii
focus; foci
fungus; fungi
formula; formulae
alumnus; alumnae; alumni
spectrum; spectra

Note that the Latin plural "stadii" has been dropped for the English plural: stadiums.

AlpheccaStarsNote that the Latin plural "stadii" has been dropped for the English plural: stadiums.

Um, isn't the Latin plural "stadia"? I sometimes hear and, I think, even use "stadia".

"Foramen magnum" means "big hole" in Latin, but in English it's the anatomical term for the hole in the skull through which the spinal cord passes. No native speaker of English would call a large pothole a "foramen magnum" except as a bad joke. As a technical term, the phrase retains Latin's placement of the adjective after the noun.