Monsters are one of my favorite things to design for D&D 5th Edition. They’re what got me into game design in the first place. They’re also the most-codified thing to create, with extensive rules on how to design your own monster in the Dungeon Master’s Guide (the rules are inconsistent and bad, but there’s at least more there than for designing a class or subclass), plus plenty of other resources out there to help you in your design process.
When I start designing a monster, the first thing I come up with is a general concept. What makes a good concept is entirely subjective, so I tend to come up with something that I think is cool or interesting. Don’t be afraid to borrow ideas, as long as you put a unique spin on it so that it’s not outright theft. Be open about your inspirations, both to protect yourself and to give people a place to look if they like your borrowed idea. I’ve recently been playing a ton of Monster Hunter, so that’s likely to influence my creative decisions. In fact, I’ll use Monster Hunter as an inspiration for this creation.
I’m inspired by the early game large monsters of Monster Hunter. They are much more complex than the small monsters you’ve encountered and present a threat to the hunters, but they still are limited to a couple unique abilities so that players aren’t overwhelmed by them when they’re first picking up the game. Still, I want to distinguish this creature from a typical early Monster Hunter boss though its appearance. Early Monster Hunter enemies are typically bird wyverns (essentially big velociraptors) or fanged beasts (bears, boars, monkeys, etc.). My first idea is a large amphibian, with elements of frogs and salamanders. It has a large vocal sac like a frog, which it uses as a weapon to direct a shock-wave of sonic energy at its prey, but otherwise it’s a straightforward fight with maybe a few hopping moves or something like that to further reflect its amphibian nature.
So just like that I have my concept, but specifically for D&D a concept needs to be accompanied by its expected challenge. Some people might figure this out as they go, but my personal design philosophy is that each monster occupies a specific space in the game’s ecosystem, and that space determines how tough the creature will be, so a general level of toughness should be determined before anything mechanical has been figured out. This creature is pretty low on the food chain, but it still is essentially a “boss fight,” so I figure the creature will be in the Challenge 3-4 range, but I could see it being as low as 2 or as high as 5. Giving it a 4-level range means I have a lot of wiggle room, but also some constraints.