Sweetheart is a Polarchuck that "belongs (or belonged) to The Neopets Team." He was introduced on April 6, 2007 as a way of humourously promoting the new game Snowbeast Snackrifice. In that game, Petpets are sacrificied to a Snowbeast to protect a local village from an ancient curse. The Neopets Team started the news by claiming that they had not seen Sweetheart in quite a while. They mentioned several more times that they were looking for him and that they'd appreciate it if anyone who would see him would tell them.
While every mentioning of Sweetheart was merely a joke, some people went looking for him thinking it was part of a new plot. A couple of people even posted (fake) screenshots on the Neoboards of them finding Sweetheart. TNT clarified in editorial 286 that Sweetheart's disappearance indeed just was a fun way to introduce Snowbeast Snackrifice and that he in no way could actually be found.