Personally, I find that creature vocals are one of the most challenging things to creature. Alexandre is on the same path I would like to touch on, however maybe elaborate a little further.
To me, creature vocals boil down to two specific, but broad concepts - that of dialogue editorial, and that of vehicle sound effects editorials:
When it comes down to the essence of a creature vocal I find that I have to dissociate it from being a sound effect event And instead look at it as a dialogue event - instead of being English, it's just gibberish. The thing about dialogue editorial experience is that it develops one's hearing sensitivity toward the nuances of the voice which most of us take for granted. Nuances like cadence, inflection, rhythm and so forth. This all converge on the term 'emotion', because each of these characteristics, among others as well, serve as a vocal body language to express to our peers what we're feeling (such as downward pitch for sadness often, abruptness for surprise, wheezing for pain or strain, and so on). The challenge is figuring out how to lift the emotive abstract from spoken language and imprint that onto a combination of sound effects to create a voice. The translation is not an easy one, but this is where I believe having background experience in dialogue is key - because it has taught you how to properly critique the voice in abstract (as in, critiquing and analyzing the 'sound' of the voice, not the 'content' of what's being spoken).
Now what could creature vocals have to do with vehicles? Vehicles are a precision process. it's all about using the right combination of source, but careful to not create a "doubling up' of multiple engine types (so as to ensure the engine sounds as if it's a singular unit), and blending in and out of multiple source files at the correct pitch cross point so the move is fluid. This is what I find applies to successful creature vocals. The difference between 'blurry' creature vocal elements versus those which have had great care taken to their alignment a la a car engine edit is a night and day difference. The difference is that when all of these vox elements are careful working together as one unit, the vox feels to us as being a believable organic entity which is emotively expressive. It doesn't mean all elements of a growl should be aligned perfectly, but understanding the cadence of a growl - the pitch arc and roll from throat at the top to guttural at the tail (just one example). So much like cutting vehicle engine rev out and away, it's about selecting the right combination of elements which blend to create this arc, but have the illusion of being one sound
When it eventually comes to cutting these emotive creature situations, this is where the dialogue background comes in - by knowing what the abstract emotive quality is that you want to express, knowing through dialogue editorial experience how this emotive quality is imparted through dialogue/vocals, and thus being able to analyze source material to find what intuitively feels right. Sometimes you can find some stuff which hits the emotion on the nose, other times you may spend a few hours throwing things at the wall to see what sticks. Sometimes even, you have all the right elements, but all it takes is some careful nudging to hit that suite spot where they marry perfectly. For me, I work intuitively on this stuff in terms of the actual building - trying to reach that point where I'm connecting with what I'm hearing like sharing in the pain I feel for this creature whose dying and choking/growling lightly in the process. Sort of like cutting a fight - you try to build up your elements so that what you're hearing in the edit feels like it's hitting you square in the chest. Same sort of idea for creature vocals - when you find yourself beginning to empathize with what your hearing, you're beginning to give that abstract emotive a grounded, sonic identity.
And this building part is where I feel dialogue further comes in - being able to listen to what you've cut, and think in terms the rhythm of breathing, whether this particular moment is missing a vowel like roundish element, whether the growls sound great but stale (because maybe we need some air-like larynx texture just to help glue the percussive/vocal elements together to give it a natural air-moving vocal chord quality). Basically, being able to do a mental spectral break down your final sound in your minds eye into the parts you think you need to achieve it (air, throat, chest, etc).
So I believe dialogue editorial experience will help you to analyze vocal emotion in the abstract, and vehicle experience will help you to be able to articulate these emotive abstracts into a final creature voice.
Just my 2 cents.