My main sources of inspiration is the macro photos, rainforest insects, plants, and underwater critters that are Not-Fish. Brought and remixed together it can create some truly alien-looking things.
IMO to get the best mileage out of their uniqueness it is important to not just carbon-copy them, but instead, find traits that are most intriguing, and then fuse and mix them together in a new creature (while trying to avoid the "Lego anatomy" approach of just sticking parts of one creature onto another), probably also trying to adopt it for the new environment (when taking something aquatic and putting it on dry land for example).
The mistake I see far too often is when people basically just find a cool bug, enlarge it to human size, make it roar, and consider the job done. Oh, maybe also repaint it if they feel daring today. But a good design always borrows from at least four different sources.