In a campaign I ran, the world was hit by a weird zombie plague, similar to the Flood from Halo.
The plague had a peculiar effect on pregnant women, causing the baby to grow quite large in the womb, and a baby to grow within its body as well. This continued, until the creature was essentially a russian nesting doll of smaller creatures.
It would sit alone, staring into the distance. If disturbed, it would attack, slashing with claws and biting. When killed, the baby would tear itself from the corpse, itself pregnant, and attack the PCs with half the hit points of the original.
When that one was killed, another, with half again as many hit points, would rip itself free. This continued until the final baby, with only one hit point, reached a single withered hand from the last corpse, before expiring (I didn't see the point in fighting a 1 hp creature when they'd just fought one with 2 to 3 hp).
It was quite effective, and buffed by the fact that the round it took for it to emerge forced the PCs to make a will save or be frozen in horror and disgust. It only worked once, but it was a good creepy encounter and they told me later they liked the idea.