Hamaguchi just mentioned on Ramuh Summon:
I’m happy with the final designs–-although I do have one minor regret. We actually originally wanted to include Ramuh as a Summon—fans of the original game will remember him fondly!
Unfortunately, we had to give up on the idea for two reasons. Firstly, we thought there would be fans who would like to use Leviathan in Mako Reactor 1 like you could in the demo for the original game. Secondly, we also thought that fans would be disappointed if they could not see Bahamut in a Final Fantasy game—I know I would be!
So unfortunately, Ramuh doesn’t appear in the game – but he’ll definitely be in the running next time we make a decision on what Summons to use!