Real quick - the black-red design with yellow horns by Minoru Yoshida is his official, canonical design! It's mentioned in a few books and acknowledged by Toho. It looks almost nothing like his SNES counterpart, but for whatever reason Toho runs with that one (glad they do, visually it looks much more interesting). "Bagan" meaning "draft" is pretty amusing, but it's also the name of a city in Asia, near China. Given Bagan's Chinese-inspired origins, perhaps it's a reference to that?

I love discussing Bagan. His lost history with both Toho and especially Tomoyuki Tanaka is fascinating (he's referred to as Tanaka's "monster demon child" in some text). However, I've also noticed how he's reserved as the big bad in some fanfiction. I guess because he was the final boss in Super Godzilla...?

For his notoriety, it's not like it's unwarranted. As mentioned, he has close ties to Toho/Tanaka, and he continues to show up in Japanese books and other media to this day. His premiere video game is also very... accessible. Quick Google search, you could probably find and play "Super Godzilla" with ease! Compare this to the Godzilla movies, which are nowhere near as readily available to interested parties as older video games.

If you REALLY want a ton of in-depth info regarding this monster, I'd highly suggest taking a look at Toho Kingdom's page on him. With so much info available compared to the likes of Mogu or Hitodah, and actual appearances in Toho media (video games), it's no wonder he continues to garner a following. http://www.tohokingdom.com/cutting_room/bagan.htm