User blog comment:Lord Loss/Monster Hunter Theory... Blog/@comment-575012-20121009212211/@comment-575012-20121010184304

@Dax: I actually considered hydrogen! Problem is, not only is hydrogen hard to be contained, but it is absurdly volatile! Yama Tsukami does have that fire cloud attack, but were it fueled by hydrogen, it would be something more like a huge explosion! Methane is also volatile, but much less so than hydrogen. I also considered helium, but it isn't the byproduct of any biological process. That's why I turned to methane: it is easily produced by biological process and, while not nearly as light as hydrogen, is lighter than air, and would make a body float were it present in enough quantity. And, after all, Yama is a tentacled balloon! =D

Come to think of it, if methane's density is the same as ammonia, that would also make the latter a suitable "floating gas"! But then Yama would have to have a poisonous attack, I guess XD