one way is to analyze monsters into their separate features, e.g.
overall body type (snake, shark, blob, humanoid, reptilian, bovine, spider, politician, etc.)
torso if any (chest, hips, and spine can be varied)
feet, hooves, toes ?
tail(s)
eyes
ears
horns
claws
mouth or whatever 
method of breathing
tentacles
suckers
antennae
fronds
teeth
mandibles
barbs
scales
fur
tools
clothing
fashion accessories
tattoos or brand-marks
guitar
gears and pistons
baked goods
etc.
and then for each of those, think of different variations
then mix-and-match 
try changing the proportions of different features in relation to the rest (e.g., huge eyes, tiny ears) – or even omit a feature for a creepy appearance (e.g., no mouth or no eyes)
also look at pictures of different critters for ideas, e.g. photo books, magazines, the Google image search or nature shows on tv – don’t forget bugs, insects, sea creatures and microscopic lifeforms
be willing to doodle out dozens of potential monsters to choose from
the scene might influence the development of the monster, or the monster might suggest the environment
might as well put some variety into how the scene is staged, e.g., do they both have to be standing on the ground? couldn’t one be hanging upside down, or oozing out of a window?
take away all the protagonist’s usual weapons and then see what happens
use paper – even just for rough sketches – lots of artists past and present work out at least the outlines of the composition and cropping before working in the final medium
have fun! 