A Base Epic Monster is a classification of monster that, at its lowest level of evolution (officially the Stage 1 form), is an Epic Monster.
Base Epic Monsters on this wiki are categorized such that while a Scorrode is an Ultra Monster itself, and Necronubis is a Legendary Monster, both are Base Epic Monsters since their Stage 1 forms, (Spee and Inpu respectively) are Epic.
All Base Epic Monsters evolve into Ultra Monsters in their 2nd form, and Legendary Monsters in their 3rd and final form.
Also, all Base Epic Monster's Stage 1 forms are tameable with a Taming Gem although only at a maximum chance of 15%.
A list of Base Epic Monsters can be found on their category page here.