Ah, yes. Stitch and Toothless. They have a few things in common. For one thing, the movies they star in are directed by the same guys, Chris Sanders and Dean DeBlois. And another similarity is their designs.
Moving on, you know how sometimes people compare characters like Ariel, Belle, Jasmine and Tiana based on animation designs? Well, perhaps that's because they're animated by the same person, and thus look similar to one another. Stitch and Toothless are no exceptions.
For instance, you can tell that Stitch and Toothless look similar from their melon-shaped heads, their big eyes, their noses and claws, and their teeth. Just look carefully.
Stitch (C) Disney
Toothless (C) Dreamworks