Einstein is a little piglet who Megan raised and made her pet from the episode, Pigs.
He is supposedly one of the pigs from the pig pen at Megan's family pig farm, and he follows her to school along with all the other pigs when they wreak havoc, and he steals the exam math test results and helps her pass with perfect scores, and giving her lots of respect throughout the school that day, despite being one of the pigs that caused some of the havoc. He gets named, "Albert Einstein" by Megan in honor of his smart side, but "Einstein" for short and gets adopted by her.
He is then seen throughout the episodes helping her, and being by her side when she goes on her adventures now., including to the zoo. It is unknown what became of him after this.