This is probably true, however I really don't think he looked fierce to begin with. It's a big blue turtle thing that summons a puddle of water around him.
I think they should have selected some other monster for inspiration besides a turtle. Look at him. He looks almost... silly.
That said, I lean mostly towards the FF IV fiends, but not all.
Earth Fiend: Lich - He looked really cool, even in his old NES sprite design.
Fire Fiend: Rubicante - He was just a cool guy, and sometimes a villain that has a sense of honour, dignity, and grace with his power while showing a good level of intellect [while still being quite evil] is a refreshing change from the brazen, ferocious, rock-brain monster brutes [these make for great monsters, but there are an awful lot of them].
Wind Fiend: Valvalis/Barbariccia - She just looked cool, and I might be biased because I appreciate seeing women proving themselves in some way. Although Tiamat does indeed look very awesome, something about Valvalis wins me over [and for the record I'm gay, so it has nothing to do with the fact that she's almost naked].
Water Fiend: Kraken - I think a killer octopus wins out against a turtle in terms of looking ferocious. Though I have to say that his NES sprite looked "meaner" than his GBA sprite somehow.