Newspaper Dagens Nyheter (DN) reported that several hateful and threatening remarks have been posted on Flashback and other social media sites.
"Expect a visit BASTARD!" and "I hope he will suffer a lot until the day he ends up in hell" were among the comments on the Flashback thread, which were subsequently erased on Saturday.
Another online commenter asked if one would be convicted over incitement for writing "Authentic images of [the suspect] with a ripped-open stomach wanted: 150k".
The 16-year-old was arrested on Thursday on suspicion of killing nine pet cats in the Gothenburg area. He admitted to the crimes during questioning.
As police had suspected, the teenager lived in the residential area where the cat killings occurred. Some cats had their stomachs ripped open and one was found hanging from a tree.
The 16-year-old is suspected of animal cruelty stretching back as far as November 2011, and which was carried out up until as recently as Wednesday when a cat was found dead outside its owner's home.
A cat was also found dead behind a grocery store in the Gothenburg area as far back as 2008, but no arrests were made then.
The 16-year-old's lawyer Göran Franzén told DN that he was unaware of the online threats directed at his client.
"This is news to me and I spoke to both him and his relatives as late as Friday evening and it was not mentioned then," said Franzén.
The police investigating the pet killings received help from a group of cat lovers who organized themselves after meeting online on the Flashback forum.
According to information provided by the group, two cats were also found dead in 2009 and 2010, about a year before the time period covered by the preliminary investigation.
The 16-year-old has no criminal record. Due to his young age he was not detained but was taken into care at a youth home.