Octopuses have three hearts and blue blood.
Two of the hearts pump blood through the gills, while the third sends it around the rest of the body. When an octopus swims, the body heart actually stops beating, which is one reason they prefer to crawl.
Why is this true?
Octopus blood uses a copper-based protein called hemocyanin instead of iron-based hemoglobin. Copper carries oxygen efficiently in cold, low-oxygen water, and it turns the blood blue.
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More about animals
Animals are the easiest place to find facts that sound invented. Three hearts, blue blood, cube-shaped droppings, a punch fast enough to boil water for an instant — every one of these is ordinary biology to the animal doing it. The collection here leans on well-documented behaviour and anatomy rather than folklore, so each entry comes with a short explanation of why the trick actually works and a source you can read next.