itch.io

They can be used in a game you made and sell but not if what you're making is a monster catcher creation maker, as the assets are licensed to you, but what you're asking is basically if you can sell licenses to the assets to other people and profit off of it. Those people/users of your maker need their own license of these monsters.

However, if you really want to use my monsters for this, something you could is use the monsters in your maker in images, screenshots, marketing, etc (as long as art is publicly credited back to us), but not included them in the base engine. Then, provide a way for people to purchase my asset here to obtain their own license to these assets, as well as a method or instructions to easily then import the monsters into your engine to be used.