Parasite are very unusual creatures, their entire livelihood relies on an unwilling host to allow them to live, some parasite even control the creature itself! Despite this, to my knowledge, there are no parasites that are very intelligent, never mind sapient.
My question is what sort of parasitic species design could possibly develop sapience, and what forces would encourage and lead to that development?
While we might not see that exact behavior on Earth, it is quite reasonable to imagine that it exists somewhere in our Universe.
And if we relax the definition of "parasitic" a little bit you will find that there are quite a few examples on Earth as well.
Whenever we hear the term "parasite" we get a mental imagine of some disgusting growth, or insect. However, a lot of species have codependent relationships. A parasitic relationship is simply taking that one (disgusting) step further.
Imagine, for example, that this species is a sentient jellyfish of some kind. It is incredibly fragile, and fears the predators on its world. How it evolved sentience and sapience is beyond the scope of my answer - it simply happened (and to counter SpaceLizard's comment, some creatures are quite intelligent while having quite small brains - although they are not sapient. We do not have a 100% correlation between brain size and sapience/intelligence).
In order to protect itself it attaches itself to a larger animal. A shark style of fish, for example. Or a whale. Over time, some of these creatures find that they can extend tendrils into that specie's nervous system, and tap in, allowing it to somewhat alter its behavior.
As the species evolves, some individuals will become wildly successful at doing this, and slowly but surely the entire species will end up being able to control their hosts to a larger degree.
Fast forward a few hundreds of thousands of years, and next thing you know these creatures are able to control a whole range of creatures with a certain nervous system structure.
Thus, the forces that would lead it to develop this ability would be the same as what's driven the evolution of every other species: a better chance at survival.
SpaceLizard has pointed out that your parasites would have to be big to have an intelligent nervous system, and therefore the hosts would have to be much bigger.
But what if a parasite depended on a group of hosts, rather than just one?
What if that parasite at one stage depended on one host, and then on another at another stage?
Such a parasite could develop complex life strategies, which in turn would tend to increase its intelligence, making it capable of even more complex behavior.
It could even deduce that, being successful by parasitizing multiple hosts, it could be even more successful by cooperating with other parasites.
So for instance, during the first stage of its life it could live within the body of its parent. Next it could live by ingesting fluids from several host bodies where no single one would be large enough to sustain it. Later on it could live on the products of several smaller hosts for an indefinite time, while itself breeding.
It might even become intelligent enough to supplement its diet from other sources, and learn to process the products of the living beings it bred on.
By cooperating with other such parasites, it could perhaps develop division of labor.
And such parasites being intelligent, it might occur to some of them that they can be parasites at second hand, living off other parasites of their kind.
Hey wait a minute - fetuses, milk, eggs for breakfast, farmers - politicians - bankers -- sounds like Homo sapiens!
Sometimes it seems like Nature has answers for all our questions, whether we like the answers or not.
