The party crosses a swamp deep in the woods. A collection of two types of carnivores and three types of herbivores drink from the swamp and wander around without fearing or eyeing one another. The creatures can be anything that fit the setting and challenge rating of the players. I’m going to use dinosaurs as my example creatures.
As the players draw closer, a pack of small velociraptors attack (the actual 3 foot tall kind, not the 6 foot Jurassic Park kind). These creatures work to surround and attack the players, attempting to use their numbers to overwhelm. However, these creatures should be weak and easily defeated by the players.
Grant the players a check (perception or knowledge nature based). If they succeed in the heat of battle (or before battle), the player notices the eyes of the raptors are strange. Their eyes look goopy, their pupils and iris’ slouching. A small secretion of something leaks from both of their eyes as well.
When one raptor takes damage (bringing it close to half health or lower), it and another raptor will press against one another. Their flesh bows and stretches before the two creatures start to mush into each other. The creature grows a bit in size, their combined mass bulking it. Not all limbs, tails, heads, or body parts remain seen as the new body envelopes different parts at random.
The wounded raptor limps toward its still healthy companion. Its clawed toes scrape across the ground as it pushes itself against its companion– until it falls into its skin. The wounded raptor’s body mixes with the flesh of the other, nearly swimming within it for a moment before the two beasts settle. The new raptor is slightly wider than it was the moment before, with the bulk of the wounded raptor filling its form. Two small arms flail uselessly atop the raptor’s back. A third leg springs from its side, swiping manically at whoever draws close. Only a small portion of wounded raptor’s snout sticks out backwards from the side of the other raptor’s neck.
The new creature gets HP equal to the combined current HP of both raptors, even if it is higher than their max HP on their own. The new creature also gets an additional attack based on a part of the weakened creature that molded with the stronger. If its head remains exposed, the new creature gets a second bite attack. If its leg remains exposed, it gets an additional claw attack. It can have more limbs exposed, but it only gets one new attack.
As more raptors are injured, they will mush together. There is no limit to how many can do this, and you can have the new creature gain one size category when you see fit.
As the players are fighting the raptors, some of the wandering herbivores move toward the party and attack them. In my case, several not-fully-grown centrosauruses and charonosauruses. These creatures should be more difficult than the raptors, but still not too hard to defeat.
With another check, a perceptive or knowledgeable play could spot that the centrosauruses and charonosauruses have the same eye condition as the raptors.
As the herbivores join the fight against the party, the amalgamations continue to grow in number. Already fused and not fused creatures of differing species can mush together, taking on the main form of the largest of the two. It always gains one new attack with each fusion as well as combining the current HP of both creatures. The new amalgamations can have more than just attacks from the fused creatures. If the creatures you chose have abilities or skills, such as scent or a gaze attack, the new creature can have those abilities in addition to a new attack.
The creatures will dwindle in numbers but continue to fuse together creating a larger and larger twisted collection of reaching claws, snapping jaws, and horns. If the fight is still too easy or to really surprise your players, have the final mashup fuse with an even larger creature, like a full-sized stegosaurus or triceratops.
The point of this encounter is to surprise the players with an ever-changing enemy and to give them an enduring fight that, while never extremely difficult in any one part, becomes difficult because of the changing aspects and how long it lasts. If the fight is too easy or if you need more obstacles, you can make the swampy ground difficult to move in, or have some creatures hiding within the swamp itself.