Great question!
Behemoth work is some of the most daunting (no pun intended, ha) work in Dauntless as behemoths are not only the main focus of the game but also brimming with personality.
The first thing I think of when beginning to prototype the sound, or more specifically, the vocalizations of a particular behemoth, is to look at the overall visual design of the behemoth and determine what qualities would best suit it sonically.
With certain behemoths this process can be quick; take Torgadoro, for example. There's no doubt he's a giant ape, so naturally we went with a more humanistic approach with lots of pitched-down human vocals and gritty roaring elements sprinkled in there.
Then you have Behemoths like Thrax, where it's not obvious off the get go what exactly he would sound like. It takes a lot of prototyping and experimentation to land on something that just feels right. Lots of testing and bouncing off coworkers to see if what you're conceptualizing is helping to bring the Behemoth to life.
To answer your second question, most definitely yes! We have libraries of sounds here that were recorded to be used to help us when designing Behemoth vocalizations. Things like animals squeals, corrigated cardboard, creaky dressers, the list goes on really.