Actually Caligula and Caligulae are both feminine form, one the nominative
case, the other the genitive or possessive case - which is kind of strange,
now that you mention it, since Caligula was a male!  His original name was
Gaius, with the "us" suffix, which is the commonest format for male names
(Tiberius, Julius, Octavius, Theophilus, Petronius, etc.) and also for male
nouns (Conus, Trochus, Nassarius, Fusinus, Cantharus, Hippopus, etc.).  The
"a" ending is the usual form for female names (Elena, Maria, Helena, Julia,
Gloria, Laura, etc.) and also female nouns (Cancellaria, Tibia, Cypraea,
Charonia, Lima, Terebra, etc.).  But, when the genitive case is used,
designating posession, the male "us" changes to "i" (lamarcki = belonging to
Lamarck; reevei = belonging to Reeve).  And, the female "a" changes to "ae"
(helenae = belonging to Helen; Voluta lindae = Linda's Volute).
So anyhow, I don't know how Caligula got an "a" on the end of his name.
Seems like it should have been Caligulus.
Paul M.