Some motifs show up oddly often in tessellation art. Why?
And, what rules should guide you in choosing an easy motif?
When you choose a motif for your tessellation, choose something exotic with a shape that's got vagueness, variety, and flexibility.
If you're drawing a fish, choose one with big floppy, twisty fins. Keep the species vague or unknown, so you can alter its appearance without people saying "Hey, that's wrong!"
If you're drawing a person, use big loose clothing, hats, any kind of hair... and let him or her carry things. All these add to your choices, so you can make your tessellation outline fit.
Avoid rigid things like toasters, cameras, boxfish, robots, animals with shells or antlers, and cars. If your choice is limited by that kind of rigidity, you can't easily tweak your theme's outline. That means your tessellation is less likely to fit together smoothly and believably.
Exotic and vague things are better themes than well-known things, because your audience is less likely to say "Hey, that's not the right shape!" So, monsters are a better theme than deer; generic fish are better than a specific species, ghosts are better than robots.
Good luck, and have fun!