Why is a 12 sided shape called a dodecagon?

Answer 1

It comes from Greek:

"dodeca-" is a transliteration (writing using similar sounding letters in the Latin alphabet) of Greek for 12 - δώδεκα

"-gon" comes from the Greek word γωνία (gonia) meaning corner (or the related word γωνιών (gonion) meaning angle).

So a dodecagon is literally a twelve-cornered or twelve-angled shape, but we tend to count sides rather than vertices and so call it a twelve-sided shape.

I guess the reason that Greek is used to name shapes is that Geometry was first studied by the Greeks.