Animanga is a portmanteau of anime and manga. It's a quirk of anime that it's only rarely an original work with no source material. Western animation often has a pilot grade source but it rarely has an entire series behind it. Anime on the other hand can be based on light novels, video games, visual novels, but most often it's based on manga. This would make it little different from most super hero cartoons except that manga and its derivative anime tend to be pretty tightly linked to its source material at the level of plot and continuity. Nobody really cares what Superman was up to in Action Comic #1. He couldn't fly and his personality was pretty different. In contrast most manga have the same writers and artist for their entire runs meaning the personality of the characters and theme have a pretty high level of continuity across the life of the series. This continuity typically carries over to the anime with the expectation that most if not all of the content will be copied scene for scene if not line for line. Consequently, any statement about the events in a manga is typically true for its anime and vice versa. All of this means that animanga is a fairly natural category that doesn't take a lot of mental gymnastics to grasp or apply. The mediums can be treated as a single media ecosystem where anything that acts on manga as a whole can be assumed to apply to anime.
All of that said, animanga is a mostly archaic term by internet standards with its height of popularity in the mid 2000s. For all that it makes sense as a term most people just watch anime and don't care where it originated from. A quick check on google trends shows that animanga never got even one percent of the searches of anime or manga respectively.
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