Apparently, the English word "Herculean" has an old spelling variant "Herculæan". This seems to correspond to a Latin variant of the adjective "herculeus/Hercŭlĕus" spelled "Herculæus" (example: "Magnetem qui Herculæus vocatur ubique dispersit, quo nullus est admirabilior, aut utilior lapis in rerum natura." –Universæ naturæ theatrum, Jean Bodin, 1596).
I am a little confused by this; I think I understand the formation of the more commonly found "Hercŭlĕus" (correct me if I'm wrong, it seems to be "Hercŭl-" as in "Hercul-is" or "Hercul-em" + the adjectival suffix -ĕus), but I'm not sure if "Herculæus" should just be considered an error, or has some basis to it that I can't currently see.
I know that in medieval Latin, there was a lot of interchange between spellings with "e", "æ" and "œ", so a word might be spelled with any of these with no regard to its etymology. However, I am not entirely sure if that's the whole explanation for the use of "Herculæus" as an adjective.
I know that there are a number of Latin adjectives that did end in etymological "aeus", ultimately coming from Greek adjectives in -αιος (such as trochaeus from τροχαῖος). As far as I can tell, the "a"/alpha in -aeus adjectives originates from the thematic vowel of the first declension, but what I don't know is to what extent this suffix may have been able to be applied analogically to nouns of other declensions. (Also, I know there were similar adjectives in Greek terminating in -ειος, Latin -ēus~ῑus,, which seem to be formed when an "e"-like vowel came before the adjective suffix. So the Greek name of Hercules, Ἡρακλῆς, was the source of the word ἡράκλειος that became Latin Heraclius.)
In spoken Latin, at least, I would expect there to be a difference in stress between "Herculĕus", stressed on the antepenult, and "Herculaeus", stressed on the penult. But maybe these rules aren't relevant when dealing with medieval Latin. I was wondering if if the "ae" spelling is used in any texts from a period when Latin was still a spoken language, and if so, if that should be taken as evidence for analogical extension of -aeus as an adjective suffix, rather than just variation in the letters used to spell the -ĕus adjective suffix. Also, if there are other words where "-aeus" is suffixed to a non-first-declension noun like this, I would be interested in knowing about them.