Ultraman Belial was first introduced in the movie Mega Monster Battle: Ultra Galaxy Legends

What was Belial's first series, either his first series appearance or the series he was mentioned in?

That's both the first time he was mentioned and his first appearance. He would appear in Ultraman Zero the Movie as well as Ultra Fight Zero which, as far as I'm aware of, where his only major appearances prior to Ultraman Geed. Geed was the first time he actually appeared in the tv show.

He's also sort-of-not-quite appeared a few times; a copy of him in the Ginga S movie and his power was used as an Ultra Fusion Card by Orb and as a R/B crystal by Ruebe

Is there an online resource, that isn't a Wiki, which documents many of the Ultraman Kaiju throughout franchise history?

That's really what a wiki is for.

Half-true. There have been numerous P. R. Fansites which note the monsters of the franchise, and I hoped that an Ultraman fansite would do the same.

I think the movie was where I first saw Belial, it's about 10 years old? (I think)

https://www.youtube.com/watch?v=W-BvS0aHVlg

Another Ultra Question(s):

- Is Zero: The Chronicles a better re-telling of Zero Movies?

- When was the last time the Ultimate Force Zero seen? All I know is that they are modernized versions of other Tsuburaya characters (much like how Kamen Rider Movies have used alternate Toei Heroes as modern heroes/villains).

They were in the Geed movie, but they don't really do a lot.