A doggo is a dog, most often a cute one.
Other internet-based names for dogs include “pupper” and variations of it. It is common to see posts about cute doggos, puppers, or good boys. Intentional misspellings of words are often part of pet-related posts. They’re used to convey a tone of cuteness and to mimic the way animals might spell words if they were semi-literate.
"Doggo or fox?" pic.twitter.com/xhrObirBFr
— Fluff Society (@FluffSociety) October 10, 2018