The only monster I have in my party is Worker 8, and I hardly use him because of his incredibly slow speed. (I can't deny that his immense power and various skills come in handy, or that that you need him or a Tiamat to get Escutcheon II and Javelin II).

I wouldn't mind a high-level chocobo (of any variety) or maybe a bone snatch.
I would also consider a bird for their range; I guess my choices are because I'd only use them for support and not a part of my main force, which is why I wouldn't got for a dragon or hydra.

However, are Reis's Dragoner abilities worth keeping a Dragon around?