It has recently been suggested that Phronima was the inspiration behind the Xenomorph:
- Meet Phronima, the Barrel-Riding Parasite That Inspired the Movie Alien
- Deep-Sea Creatures That Inspired the Alien Series
From the Wikipedia article concerning Phronima:
Although commonly known as parasites, they are more technically correctly called parasitoids. Instead of constantly feeding on a live host, females attack salps, using their mouths and claws to eat the animal and hollow out its gelatinous shell. Phronima females then enter the barrel and lay their eggs inside, and then propels the barrel through the water as the larvae develop...
If the articles above are correct and Phronima is indeed the inspiration behind the Xenomorph, then it is easy to see how one might extrapolate the Xenomorph's life cycle from Phronima's.
However, Phronima's life cycle is not exactly that of the Xenomorph. For an animal using intermediate offspring to reproduce, I would nominate Cnidaria, which @Hypnosifl has already mentioned prior to my answer. Cnidaria includes jellyfish and the common sea nettle. A group of the latter is pictured below:
Here is a pictorial representation of their life cycle:
Steps 4-8 show the growth and release of the "polyp", an intermedia offspring which will give rise to the "medusa", which is the full animal.
In regards to plant species, ferns are a good example of a plant that uses an intermediate offspring to reproduce.