Scientific Parasites are a group of creatures in the Land of Ooo who feed on intelligent brain waves. This is why they are commonly found in the pockets of doctors and scientists, such as Princess Bubblegum and Doctor Princess. These parasites come in many forms and colors and sometimes have the appearance of certain animals. Dr. Donut and Betty don't have Scientific Parasites, most likely due to their lack of pockets. Science Cat also has yet to be seen with a Scientific Parasite, despite the pockets on his professorial garb.
When Finn is acting like a doctor he has two science parasites inside his coat pocket. This, most likely, is because the parasites were in the doctor's pocket before, waiting for a smart person. Another possible reason includes that Finn is smarter than he appears to be. However, the Scientific Parasites could be unable to determine who gives off intelligent brain waves and simply assume doctors are smart. Although this is highly unlikely, Finn had purposely placed them there. But, for an unknown reason, there is also one in Jake's tool belt in "Slow Love".
A Scientific Parasite appeared in almost the whole episode of "Simon & Marcy" in Simon's pocket, and seemed to show emotions, being upset after Simon defeated the creatures with snow rather than just seeming indifferent.