Disclaimer: JKR owns the Potterverse, and Viskii owns the Skittlesverse.
"That is it! I have had it with this feline!"
Lily Evans turned, startled, and saw Severus Snape holding Remus's cat Skittles by the scruff of the neck. His face was red with anger, and the cat displayed HIS frustration, constantly trying to turn and claw his captor's eyes out.
"Please, Severus, calm down! He's just a cat!" Lily pleaded, trying to save the poor creature.
Severus visibly forced a calmer expression onto his face. "Fine! Take the bloody thing before I defenestrate it. And keep it away from me!" With that, Severus stalked out of the tower. Lily feared for the life of anyone who encountered him, but she had other things to worry about. Such as...
"Shh. Shh. There there. It's alright, Skittles. He's gone. You're safe now. Everything is fine. Shh." Lily cuddled the cat to her chest, trying to soothe him. She did not even notice James enter the tower, nor the horrified expression on his face. With an unnaturally smug air, the cat began to purr as it nuzzled Lily affectionately.
After a rather choked noise, he managed some coherency. "Here Lily. Why don't you give that…cat to me? I'll go put him on Remus's bed." Without waiting for a response, James grabbed Skittles, and headed for the boys' dormitory. Lily could not understand why all the Marauders were in such moods that day. In her confusion, she failed to notice the door to the boys' dorm open again, apparently on its own, and quickly close again.
Inside, an invisibility cloak was slipped off one individual to reveal the form of Severus Snape. The cat also transformed into the same individual. Both Snapes collapsed onto the floor, laughing so hard tears streamed from their eyes. James fumed.
"Well," gasped the Snape formerly hidden by the cloak, "at least we drew enough attention. I doubt anyone will find it unusual that Snape and Skittles avoid one another after this."
"Hah," snorted the former cat, "only Gryffindors would fail to suspect Polyjuice. By the way, James…excellent taste. That Evans has incredibly soft skin and her scent-"
The Polyjuiced Remus had difficulty preventing James from strangling their fellow Marauder while laughing that hard.
Much later, it was Sirius, of all people, who pointed out the ultimate flaw in their prank. "Severus, confusing a prefect, especially Evans, is always brilliant, but your first prank was supposed to disturb at least one Proffessor."
"Sirius, Lily Evans was cuddling me to her chest. In friont of James."
"Scratch that, you passed with flying colors. Too bad we don't have pictures..."