So I was playing Deltarune and i have beaten Jevil, so I decided to save the game and go to sleep. But, in the next day, when I staarted the game, I was surprised by an animation of the Sleeping Dog. No song was playing, and the title was simply "DOGTARUNE." I already had finished the game once, so now I'm curious. What happened, and what does that mean? And I didn't change the game files. Has this happened with anybody else?

If I remember correctly normally you get sleeping dog when the game encounters a problem loading your save. You can replicate this by editing the save to put you in a room that doesn't exist. The game probably messed up your save file when saving.