If you want them to appear clever and cunning: it's obvious the party has no ranged capability, but can beat the pants off any warriors. The lizardfolk know the swamp well.1) The Lizardfolk that got away are from the Bird Skull tribe (known as a bad lot). What would be interesting for them to do next? Attack again? Send out more scouts? Retreat into their own space? Summon reinforcements?
So they send some ranged guys out to lure the PCs into a trap. Either a very dangerous area (Quicksand, etc), the territory of dangerous creatures (aggressive wasps, carnivorous plants, etc), or a place where the PCs could get lost easily. A lizardfolk simply stands at one spot, shoots, runs and hides. After so many feet, another one is hiding and he shoots, possibly luring the PCs into that direction.
A boasting match. "Oh yeah? Well I slew TWELVE kobolds in ONE BLOW!" Or possibly a competition of arms - armrestling, blowing stuff up, knife throwing, something like that.2) I'd like to introduce the rival adventurers more fully next session. The PC's are camping out in the swamp so a meeting would be easy to do, and I want to avoid combat but have a small skill challenge or something as the two groups jostle for superiority - any thoughts?
Or a good old fashioned insulting match.
Check out the Chillborn zombies. Change cold to poison. Done. Of course, you might want to level them down.3) Anyone got any good swamp-based encounters to bring to mind for a group of 1st level characters? I was thinking potentially of some undead, as they're always fun. Perhaps a group of long-dead zombies attack from underwater or similar. Or, heh, perhaps LOTS of zombies closing in from all around...
There's always swamp animals. Crocs. Here are some snakes. Bugs of many kinds. Aggressive plants.
Since you mentioned a Black Dragon (but likely won't use one?) you might have something like a Dragon Buriel Ground.Graveyard location