You've got a session to plan for your game. You've got a monster type in mind, but you just can't seem to find exactly what you're looking for. What'd help is a huge book filled with a bunch more monsters for you to comb through. What you need is Creatures: Complete Monster Compendium. The book is up on Kickstarter now.
From the campaign:
Designed by the creators of Shadows of Esteren and Fateforge, Creatures is a love letter to fantasy bestiaries. Its 400 pages detail more than 200 illustrated creatures using the 5th Edition ruleset of the most famous of roleplaying games. Its contents include 6 playable species, NPCs with related adventure hooks for all levels, reimagined classic creatures and brand new ones, as well as 15 archetypes, providing you with material to design new monsters for your games!
The campaign's more than 5x funded with 23 days left to go.