Hey, I read some sites on Assignments and came over the information on the Thorian. The Thorian shares some similarities to the Reapers.
On the site about the Thorian it is stated: "The Thorian is a unique creature with mind-controlling and telepathic abilities, and a massive sensory network. It releases spores into the air that allow the Thorian to control those who inhale them, using pain to control their behaviour. These 'thralls' then tend to the Thorian and obey its will. (...) Though it is intelligent, the Thorian only regards other sentient beings as potential thralls."
The Thorian can control sentient beings -- much like the Reapers control sentient beings by indoctrination. The Thoran also releases its Thorian Creepers. These mindless creatures use melee attacks and have long claws; they look much like the husks of the Reaper army. The Thorian existed on Feros even before the Protheans arrived there. It is the last known of its kind. The Thorian spends millenias in hibernation, then wakes up for a "few frantic centuries" of activity before resting again. The Reapers also hibernate for 50 millenia in Dark Space. Then they come back to wipe out all high advanced civilizations and go back to rest in Dark Space again. Javik stated that the war with the Protheans took a few centuries. So the Reapers are also active for a "few frantic centuries".
The Thorian has a vast storage of knowledge. The Reapers also contain the vast network of information of entire species in just one capital ship. "When the Protheans arrived and began building the metropolis that would eventually cover Feros, the curious Thorian studied them, likely made several of them into its thralls, and even absorbed them after they died, allowing them to become a part of its expanding consciousness." Just like the Reapers, the Thorian consumed the sentient beings and enlarged his consciousness. Well, at this point you can say that a Reaper doesn't consume any beings after its construction is completed.
The Thorian uses Shiala and her clones as his voice and fighter -- much like Harbinger used the Collectors in Mass Effect 2 to encounter Shepard. It is a miracle that the Thorian survived the last cycle of extinction. Some of the Thorian's Creepers were captured and send to Nodacrux where the scientists managed to turn the Creepers into docile laborers. The scientists could control the Creepers until the Thorian died. The Illusive Man also managed to control husks and make them docile. TIM was able to indoctrinate other beings, just like the scientists could control the Creepers behavior. The scientists were from ExoGeni. They also sent some Thorian Creepers to Cerberus. Maybe they worked together in study of the Thorian.
My suggestion is that the Thorian is an actual Pre-Reaper. Maybe it was the first race that was transformed into a Reaper. And it also has a similar shape to the Reapers. The reason why it survived the events of the Prothean's extinction is that the Reapers spared it. The Geth only tried to kill the Thorian to cut Shepard off trace. The Thorian was able to clone sentient beings which is a proof of its high advanced internal "mechanics". Unlike the Reapers though, the Thorian does not have any further interest in human beings.
"In the aftermath of the geth attack, ExoGeni took the remains of the Thorian as they pulled offworld. Unfortunately, the effects of the Thorian spores still affected its victims. The colonists began having headaches or muscle spasms, similar to what they experience while under the Thorian's control, whenever a colonist goes near a former Thorian victim, they could feel the victim's heat sensations or pain. Shiala, if spared, has her skin pigment changed to green, similar to her clones, and her biotic abilities are unstable." It is similar to the Reapers' indoctrination: Even after death (the derelict Reaper) the sentient beings aboard or surrounding it will still become indoctrinated and eventually altered into husks. Shiala was also altered for some reason.
"When the Reapers attacked Zhu's Hope, the lingering hive mind effects of the spores allowed them to coordinate effectively." This refers to the colonists who have almost developed a collective consciousness -- a reference to the collective mind of a single Reaper?