It wouldn't surprise me if some of the unseen Titans (as well as the ones we've seen, thus far) where nods/similar in appearance to some of the least used kaiju. Specifically, (and please bear with my headcanoning, here)...
Osoros >>> Behemoth
Gyottos and Eartheater >>> Methuselah
Arkitius (and maybe Crustaceous Rex) >>> Scylla
Seaweed Monster (Kaiso Kaiju) >>> Amhuluk
Odako/Giant Squid >>> Na Kika (and kinda the Mire Squid, but it's not a Titan)
Umihebi/Giant Sea Serpent >>> Tiamat
Korghan/Bagorah and Giant Bat >>> Camazotz
Kuroiga/Black Moth/Griffon, Norzzug, and Stone Guardians >>> Sekhmet
Okondoru/Giant Condor and Quetzalcoatl >>> Quetzalcoatl
