Titan World : Book One Leitmotifs

Quetzar : Slow strings and vocalizations punctuated by church bells

Kedos Croder : Powerful brass mingled with deep, ominous vocalizations 

Grinnin : Subtle string work underlain with tribal-sounding percussion

Drakia : Fast-paced percussion and strings with notable oriental styling

Vanyris : Shrill, piercing strings and whiny brass in up-and-down melody

The Sub-Titan : Soft strings heavily segmented by sudden, pounding percussion

Plume : Loud, heroic horns that would not sound out of place in a 70's sports movie 

Fainis : Thumping, dominant vocalizations and string work

Strider : Guitar riffs mixed with frantic strings

I'm imagining lots of cool things with these such as a theme interrupting or overpowering the current music when a new monster enters a fray or two pieces of music mixing together when monsters team up. Kedos and Quetzar in particular would benefit from that last one, I think, heheheh. And Strider would be a good candidate for the former, the big ol' jerk.