Why 'd' in 'Aeneid'?

The Latin poem Aeneis is Aeneid in English. How did the last d come about?

A few suspects by quick search:

• /ð/ → /d/ shift in English, but there must be a shift /s/ → /ð/.
• It seems romance languages write it with d, possibly it came from French? (German does not have d).

Latin words have different forms for different grammatical cases (depending on their function in a sentence or phrase). Aeneis is the nominative case form of the title in Latin, but the stem is actually Aeneid-. The -s at the end of Aeneis is a nominative singular ending that causes the d at the end of the stem to be deleted, since Latin doesn't allow words to end in a cluster *-ds. So the grammatical structure of the word is Aeneid-s which ends up as Aeneis. When the word has a different case in Latin, it takes other suffixes and the d is visible.

When a Latin word is taken into English, sometimes the nominative form is taken as-is, but sometimes a form based on the stem is used. It may depend on the details of how the word got into English. Actually, the Oxford English Dictionary says that in the case of Aeneid, a third type of form based on the Greek-type genitive singular (A)eneidos was once used in English.