The whole 'Creating a Monster' section in chapter 9 of the Dungeon Master's Guide is worth the time to read and review regularly when it comes to customizing monsters. The Dungeon Master's Guide also has NPC features that act as templates on page 282. Such small reskins are often all we need. In many cases, this advice alone is enough to serve as a monster template.

Need a flying monkey? Consider a baboon with wings and a flying speed. Need a fiery phoenix? Take the giant eagle or roc, give it immunity to fire, and allow it to deal fire damage with its attacks. The 'Modifying a Monster' section in Chapter 9 of the Dungeon Master's Guide offers excellent advice for modifying monsters to suit the story we have in mind: Frankly, as a proponent of lazy DMing, that's not a process I recommend. On the very far end of this spectrum we might create an entirely new monster from scratch.

We might change a handful of mechanics, giving our fire giant wight necrotic damage instead of the fire damage it might do. We might change a monster's type, such as creating a fire giant wight by giving the fire giant the undead type. We might simply call one monster by another name and description, called reskinning monsters. There's a wide range of ways we can change up or create new monsters in D&D. New to Sly Flourish? Start Here! Mashing Up Monsters: Using D&D Monster Stat Blocks as Templates

D&d making a monster