So one of the more interesting but overlooked traditions of the FF series is letting a prominent boss monster finally become a summon.
Titan was a minor story enemy you had to face in FFI to proceed but became one of the Summons in FFIII.
FFIV allowed you to summone enemy staples Goblins, Mindflayers, Cockitrices and the Bomb
Golem was a pretty common enemy in the first four FFs and finally got upgraded into FFV's most useful summon.
Cait Sith was a minor enemy in FFII and FFIV, while if you go by the original Kanji, Midgard Zolom is a late game regualr enemy in the World of Darkness
FFVII made Chupon/Typhon into a summon.
FFVIII upgraded new enemy favorites Cactaur and Tonberry into summons. It also granted summoner status to Minotaur from FFV, and Cerberus from FFIII, and its very likely that Doomtrain is a reference to FFVI's Phantom Train boss. Let's also not forget Gilgamesh!
IX gave us Atomos from FFV
FFX gave us the obvious Magus Sisters but also possibly upgraded Yojimbo who was a nasty enemy encounter in FFV and VI respectively.
FFXI finally gave Garudo from FFIII's its dues
FFXII upgraded all of the Lucavi from FFtactics into summons but also ExDeath, Zeromus, and Chaos
FFXIII gave us Hecatoncheir from FFIII, XIII's version is surprisingly closer to the real myth version of the beast instead of FFIII's version.
So with all that said, what enemy/boss would you like to see get upgraded into a future summon for the series?
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For regular enemies, I would love to see Behemoth finally become a summon, as well as the Movers from FFV. Brachiosaur would also be pretty cool.
For bosses, I'm hoping to see Hyne become a summon, Ultros would be pretty cool as well, same with Demon Wall. I would really love to see Ultima/Atma Weapon become a summon as well.