That pegs Harrelson as Cletus Kasady, the serial killer that eventually becomes the symbiote villain known as, yes, Carnage.
In comics lore, Eddie and Cletus originally met when they were both in prison. When Venom was helping Eddie escape, a little part of Venom -- an "offspring," if you will -- got left behind and linked up with the maniac, thus creating the fearsome Carnage. Because putting a serial killer and a man-eating alien together certainly means that heads will roll. (If not roll, then get devoured.)
It's unclear at this point if the cinematic version of Cletus already has his symbiote friend or if he's just alluding to the chaos he can potentially create. Still, this mid-credits teaser is a clear play for a sequel, even if Venom itself has been mildly received by critics. (The very final post-credits scene is an extended sequence from the upcoming Spider-Man: Into the Spider-Verse, which is very fun, but does not explain why there is no Spidey in Venom.)
The film sets up the idea that there are plenty of other symbiotes out there -- an entire universe of them eager to use humans as snacks. When Riz Ahmed's tech bro villain Carlton Drake's rocket brings Venom to earth, it's accompanied by three other blobs of menacing goo. Two die over the course of the movie as Drake experiments on them, one becomes Venom, and the other, Riot, links up with Drake and is ultimately defeated in a fiery blast.