While reskinning monsters, the act of using the stat block of one creature but describing and naming it as something else, is perfectly okay, there isn't always a perfect match. Dungeon masters often find themselves wishing there were more options or something more specifically tailored to their campaign, and generally the only way to solve this conundrum is to create something from scratch.
Dungeons and Dragons 5e's Monster Manual is packed full of unique, diverse, and vicious monsters for dungeon masters to use against their players, ranging from dragons and giants to hungry treasure chests, but sometimes this range of options just isn't enough.