I love ypotryll, but if you have a high level party they'll probably be little more than nuisances. God help you if you're playing mythic.
Either way maybe throw the savage mythic and advanced templates on them to male them worthy pets of the titans.
May I suggest the mighty bhole?
Hard to miss, great potential for intimidating imagery, and big enough to serve as a mount for a titan. It also fits in nicely with the whole "primordial evil" and Lovecraft themes that you seem to be using.
Also... yes. Everything is little more than a nuisance at mythic levels. I long ago gave up the idea of making viable mythic characters or monsters.
Making any sort of challenge for mythic characters is near impossible- or it's total rocket tag, and the entire battle comes down to "who can roll the d20 higher during initiative". At least if you're designing your own campaign you can give the PCs some challenges uniquely suited to them.
I'd advise slapping class levels onto the titans, and/or swapping their base mythic ranks for mythic paths like champion or marshal. This'll give them a greater pool of mythic power, as well as some nice abilities. Your PCs are expecting titans to charge into battle and smash away with their maces- imagine their reaction when the Arcane titan instead casts time stop and takes some mythic extra standard actions to spam scrolls. You could always add completely unique mythic abilities to the titans as well.
Also, the CR system falls apart completely when mythic is involved. If you haven't been doing this already, switch to milestone XP so that you can add templates, gear, and special abilities to your baddies wherever you want without having to level your PCs faster than normal.
I suggested templates due to the limited work involved. Obviously that won't bridge the gap by any means but it'll give them a little more kick.