Why and when did 'hendiadys' change from its original 'hendiadyoin' spelling?

Solution 1:

The OED’s earliest citation for hendiadys is from George Puttenham’s ‘The Art of Englishe Poesie’, published in 1589. The etymological note for the entry reads:

Late or medieval Latin hendiadys , < the Greek phrase ἓν διὰ δυοῖν ‘one by means of two’.

The Greek phrase is apparently not found in Greek grammarians, but is frequent in Servius on Virgil; in late MSS. of Servius, it appears latinized as endyadis , endyadys ; Papias (12–13th cent.) has endiadis.

Solution 2:

When

Google's Ngram Viewer shows only hendiadys showing up in English books, and it turns up around 1820:

Etymonline.com says hendiadys goes back further:

hendiadys
1580s, figure of speech in which two nouns joined by and are used in place of a noun and an adjective; from M.L. alteration of Gk. hen dia duoin "one (thing) by means of two." If this term was used by Greek grammarians it is no longer found in their writings, but it is frequent among Latin writers.

It also has no entries for hen dia dyoin or hendiadyoin.

Together, these things suggest that hendiadys is the term adopted into English rather than the origial Greek or Latin hen dia dyoin and hendiadyoin.

Why

I don't have a definitive answer, but Anglicised loanwords often take on a different form from their original and drop accents. Note the Latinised form is hèn dià dyoîn. For example, French risque is first recorded as risk in 1728 and French bâton as baton in 1540s. There are many here and many more here.