Rocket Raccoon exists within the Marvel Universe Earth-616 continuity. The character has a complicated in-universe back-story. He first appeared in Marvel Preview #7 (June, 1976). Here is the abridged version.
- Alien humanoids settle the largest planet in the Keystone Quadrant star systems.
- They build a sanitarium to study a variety of mentally-ill patients.
- The aliens build a robotic staff to help maintain the patients.
- The aliens lose their funding and leave having the robots remain behind to take care of the facility.
- The robots chafe caring for their patients and genetically modify animals to take care of the patients so they can leave for other pastures.
- Ranger Rocket Raccoon is the leader of that group of protecting animals.
And for the scientific purists among you: It is likely he is a raccoon-like organism, not necessarily a raccoon from EARTH. But for the sake of the non-scientific, the difference is minimal. Star-Lord recognized him as a raccoon and that is what he calls himself.
Consider the writers of that era, less than imaginative and leave it at that... Try not to read too much subtext into it. During that time writers were doing whatever they thought might bring in dollars. Scientific purity, aliens, and planetary genesis concepts weren't a consideration. Just run with it.