Here are my theories. Here is what the hypothetical fanfic would establish for these beasts:
Ultros & Mr. Chupon
“I’ve got more lives than I do arms!” - Ultros
Ultros can talk and cast Haste, Protect, Stone, Magnitude 8, Aqua Breath, Firaga, Blizzaga, Thundaga. We know from our time at the Magitek Research Facility that the Empire has successfully infused soldiers with magic, though these early experiments caused them to go insane (thus, Kefka).
Ultros was for sure some basic-bitch river monster that the Empire fished out, stuck in a tube, and flooded with magic to become something akin to what they’d later achieve with Number 024 and Number 128. Let’s say he’s Number 001, if that wasn’t already Kefka. Since Terra has been in and around that Vector facility since she was a baby, he’s definitely seen her at some point. This and being around the meatheaded soldiers day in and day out steeped him in a miasma of toxic masculinity of years, resulting in an octopus who knows you’re supposed to claim women but also not knowing what to do with them once you’ve done that. Presumably he learned the language to not go insane.
Mr. Chupon seems to have been two different monsters the Empire fused together in an attempt to make another Number 128. It worked, and he’s conditioned to listen to people. Since Ultros can talk, Chupon’s sort of primed to listen to him. He’s stuck with the octopus ever since.
Ziegfried
This jagoff was definitely Gogo, in the Before Times, before the World of Ruin. Their entire thing is that they mimic people. They also suck real bad when you initially fight them, unlike the real Siegfried in the Coliseum, who presumably is a badass.
I’m going to say Gogo/Ziegfried’s also Lone Wolf the Pickpocket, because there’s no other sentient werewolves in this game, and Lone Wolf also sucks complete ass at what he does. I think the person Gogo was realized they were Bad At Crime, but amazing at mimicry. Thus, you find them having focused on that over the other thing after being vored by that hellbeast.
Just a bloke who wanted to be anybody but who they were…