It spends its larva years as a floating brain just taking in data with the entirety of the brain dedicated to taking in data, with really good senses, locking in on carbon compounds, while its amoebocytes, in a thick mucus surrounding it, pull in food automatically and provide motion. Over time the amoebocytes mature, and create a connection with whatever organic thing they see, usually the one that fascinates the brain the most, to prevent it from choosing something dead. It then supplies that creature with nutrients to establish good terms with it. As it grows it pulls in closer to the creature fully realizing its neurosystems, and conceptualizing them . After a while its amoebocytes cover the other creature and it attaches to the brain of its host. As it has no wishes of its own, this is entirely beneficial to the host. It then has the amoebocytes alter the creatures body for a optimized use of its new abilities ( read: it makes it look like a psychic creature). The amoebocytes are now connected to the creatures mind, and it moves at his whims (allowing it to mimic telekinesis), pulling things with a strange green goo. It can send messages to straight to the brains of creatures it observed well-enough in its larva stage, by sending a few amoebocytes to the creature it wishes to contact. The more abmoebocytes the more distinct the message, but with just one amoebocyte it can activate the fight or flight instinct, the most basic telepathy in its arsenal. There is only one of these creatures at a time, as it creates a larva when its host dies. It cannot, of course, survive past the host since it is so intertwined with its host.
In some cases being host to the psychic creature creates dramatic change, such as happened to this pencil spider. Note, however, that originally he had large pincers, but lost those in a brawl. This host is the most document host in history. He was famed for his elaborate labyrinth of paper and webs. He died thousands of years ago, and since then sightings of hosts have been rare and largely discredited. It is from him these psychic symbiotic creatures gained the slang name "Psyders" as many did not realize it could bond to any organic being. To this day, "Psyders" remain the most common terms for these beings, as scholars do not bother thinking up a new one, as it has been over 500 years since the last sighting.
Excerpt from Where Did The Psyders Go? - "And so, the team has come to the conclusion the last Psyder died in Alvalpa, as that is where the last host was sighted (by reputable sources, at least), and we uncovered no sign of his trail after it cut off. Surprisingly, however it does not seem it was any of the fire-based hazards that seemed to kill him, but the carbon let off by Trod's fire shields seemed to draw him. We, therefore, believe he eventually tried to bond with the fire shields, destroying himself in the process. But, hey that's just a theory.