Methemoglobin was an element found in the blood of some Humans and humanoid races. It was a variant of hemoglobin, but rather than containing ferrous iron, methemoglobin contained ferric iron, which was less efficient in binding with oxygen.
The blood of the native humanoid race of the Kornak-Jabari homeworld contained methemoglobin rather than hemoglobin. As a result, their species evolved with three-lobed lungs, as opposed to Humans' two-lobed lungs, in order to draw more oxygen. (SCE eBook: Wounds, Book 2)