Doom 64 was the first (and so far, only) game in the Doom series that was not directly developed by id Software; the title was developed by Midway Entertainment, with id supervising the project.
The new Unmaykr weapon would later appear in Doom Eternal, likely due to its popularity among fans.
A new two-headed hell hound enemy was originally going to appear in the game, but ultimately didn't make it in the final product. A likely reason for why the enemy was scrapped was to avoid potential plagiarism accusations, since a game developed by Monolith Productions known as Blood was released the same year and featured an identical hell hound enemy that also had two heads.
The Unmaker weapon was first conceptualized in the early days of Doom as written in the Doom Bible, but it wasn't utilized until 64.