The Witcher series. You can download the amazingly well done fan translations at [link]
Read through the books. Playing the games that were made by the books can increase you monster base vastly as they use many fantasy creatures from Slavic mythology and some of their own.
The most memorable designs for mythos creatures come from an understanding of the creatures anatomy. Once you have the musculo-skelatal structure nailed down it imbues the image with a depth of life on the page/screen. There are plenty of 'monsters' in prehistory (the Devonian period has some nasty looking examples) to use as a starting point, for a recent piece of artwork featuring a 'hunting horror' I began with fossils of the Dunkleosteus fused it with the Coelacanth and 'evolved' the end result by a few million years into a land creature. As has been mentioned before googling the base form of the creature I was looking for produced enough reference images and video for the final artwork.
I plan on using it to come up with my own comic, so am looking for ideas on races, and other things magical for inspiration.