Until now, my stories have pursued moderate realism. Science Fiction after all requires a degree of believability in order to be a viable genre. But now that I have begun to break into the fantasy racket, I'm beset with hardship creating beautiful monsters. So let this be a quick guide to help you on your journey making better, believable monsters.
Monsters tend to be thematic. By "monster" I don't mean creatures exclusively supernatural. Aliens and fictional indigenous creatures count as well. Designing thematic creatures should be based upon a singular theme. Try not to combine too many attributes, otherwise doing so will thwart your concept. If fire is your theme, no matter what angle you approach a monster, make sure that their key concepts communicate the theme. A creature born in fire will have an ossified, rock like features, or something fluid, resembling molten lava. Water, likewise, will be cooler in color temperature, slippery, and possibly have a diverse color pallet. Follow your theme and be consistent!
Another thing that I encounter is naming creatures. In this particular case, the thematic approach works too, only this time your theme is based on languages and rooted in culture. My upcoming project takes place in a historical setting. Therefore I need to adhere to period terminology. What this looks like is simple. In my particular case I took latin and played with the scientific terminology used to name animals, then I used certain punctuation to enhance the creature names.
One thing I will discourage is taking known creatures and combining them together. Brian K. Vaughan does this in his Saga series, and to me it's the only thing that's rubbed me the wrong way concerning the series. I prefer more original takes on creature concept, but that is a personal taste of course. His work in Saga has been amazing thus far. I highly recommend it. I'm just not a fan of taking, say, a salamander's head and putting it on a human body. It's a little bland for me.
Beyond that, I would say be creative. Try to think outside the box and always be consistent in what your core concept encompasses. Feel free to share examples! I'm always willing to critique.