Deltarune, Toby Fox's follow-up to his smash hit RPG Undertale, is full of surprises and weird story twists. It's still a vessel for Fox's sometimes-twisted sense of humor. We see a prime example of an elbow-jab towards the end of chapter one with a clever reference to the Five Nights at Freddy's games.
(Spoilers for chapter one of Deltarune follow!)
When you return home after your adventure with Ralsei and Susie, you get a chance to walk around town and talk to the monsters inhabiting it. On the west side of town, you stumble upon a very ramshackle-looking pizzeria that has various mascots doing a promotion outside it. One such mascot is a purple dinosaur wearing the head of "Ice-E," a mascot character who pops up in Undertale and Deltarune (and whose name is on the pizzeria).
One of the pizzeria's costumed promoters is Deltarune's version of "Burgerpants," the highly-disillusioned burger-flipper who slaved under the celebrity robot Metaton in Undertale's underworld. If you talk to Burgerpants, he'll tell you about the co-workers wearing the mascot outfits. When he tries to describe the purple dinosaur, he says "Purple Guy—man, THAT GUY, you gotta…actually, does that guy even work here?"
Deltarune's "Purple Guy" is almost certainly a reference to Purple Guy from the Five Nights at Freddy's (FNAF) games. Purple Guy runs several pizzerias according to FNAF lore, and he's a serial child murderer. His villainous actions touch every game in the series, but his motivations are still shrouded in mystery. Thus, Purple Guy gradually became the most notorious FNAF character. No word on whether Deltarune's Purple Guy has obliterated children, but he's shifty-looking; I wouldn't go near him if I had young ones in tow.
Fox's nod to FNAF might be a way of thanking the series' creator, Scott Cawthon, who's praised Undertale in the past and recommended it to his fans. Deltarune itself is crammed with references to Undertale—though it's erroneous to call Deltarune a direct follow-up to Undertale.