First go to appdata section then Local ---> UNDERTALE -------> file 0 ------> and open with note app. Then scroll down to the bottom and select the second last number and delete it (WARNİNG İF YOU DONT WANNA LOSE YOUR UNDERTALE SAVE THE NUMBER THAT WRİTES THERE) then replace it with -1 or -11. Now if you open undertale the dog will appear. İf you want your undertale back just write the room code ROOM CODES
https://docs.google.com/spreadsheets/d/1bGnZjiSKjAjiHAwYxz09PJ1HVSCK3haCuPcjie0sfgk/edit?gid=0#gid=0