Double barrel shotgun reloading frame (Based on Battlefield 1 Double barrel shotgun secret reloading animation) By coincedence, it really looks like doom vanilla one
As I recall, it has at least been suggested (and discussed on here before) that the original idea was that the Heressiarch was actually intended as the next serpent rider (tying in very closely with the Heretic end scroller) and Korax was originally designed as his mount. Whether true or not, I don't know but, obviously, that didn't happen and by the time Eidolon came along the idea (if it ever existed) was long gone.
it could be that they were crunched for time and needed more bosses to fill all the hubs in the game, so they cut Korax in two and at that point they had two bosses to use. maybe they thought that Korax looked more intimidating and powerful than the Heresiarch, so they swapped places with the two, and because it wouldn't make sense calling the Heresiarch a Serpent Rider if they removed his mount. then when Hexen 2 came along, they abandoned the idea entirely of the "riders" and made Eidolon an even bigger monster than Korax to show how much more powerful he is.
or they could've just had a change of heart and thought it better to have the Serpent Riders be more of a figurative title than a literal one. i don't know how the development of Hexen went, and it clearly has a lot of effort put into it, so they might not have been crunched for time at all (Hexen 2, on the other han...).