Well technically Spekkio's fourth form is actually an Omicrone, an enemy that just triggers other enemies to come on screen--he can be found in 12000 B.C., but he can't be battled without modification.
There are two other appearances. The first is when escaping prison. If you free Fritz, you are attacked by an Omicrone. (or at least the same sprite model) The second occurs if you try to buy from the merchant in Madina village. Both appearances are 1000 AD.
Well, is it more probable that a creation by either Gaspar or Belthasar could unlock a person's inner magic?
I'd say yes. When searching through the various cities of Zeal, there is a craftsman striving to create tools that increase a person's abilities, ie the Tabs. Since they are scattered through time and found throughout the game, I'd say he was successful, or rather, they were successful in the past, and what success they had prior were sealed away. Various quotes from the people of Zeal reveal that they had other forms of power drawn from the planet, instead of Lavos, and these were sealed up in the north palace. If Spekkio is a creation of Zeal, then its possible that he too was in the north palace, and by whatever means, ended up in the End of Time. Perhaps his enigmatic properties, in conjunction with traveling with Gasper, it what caused the violations of space/time that triggered their arrival at the End of Time, rather than simply being flung there by Lavos.
So, if Spekkio was created by Zeal, as a sealed weapon in the days of relying on the energies planet, then Lavos does surpass him, and thus creates a limiting factor.
Now, i might be going on a limb here, but I suspect that Spekkio, and pretty much any/all of the major creations of the guru's rely on some factor or connection to dreamstone. Now, Masa and Mune are dream creatures effectively made real, and Spekkio is probably similar – both
are subject to form changes. Perhaps Spekkio's forms are then based on the 'dreams' of those around him, rather than just a singular dream.