I love designing OCs and worlds, but one of my biggest flaws is being not that great at creature design, does anyone have any tips or tricks when it comes to designing fantasy animals or creatures?

I like to take animals that already exist and mash them together. Also taking animals that exist but giving them really exaggerated shape language and crazy colors. Throw feathers on a cat or something and see what happens!

I agree with Bloodvisions; taking known animals and mashing them together tends to work really well, especially for fantasy creatures (I would make monsters in a different way though). I also like to remember big-medium-small (vary the proportions) because that makes creatures feel balanced in my opinion.