When I was younger, I loved reading the Animorphs series of books by K.A. Applegate. They were basically about a group of young teenagers who were given the ability to morph into any animal to combat an invading alien force. The invading aliens were called Yeerks and were distinct in a number of ways. They were described as a type of slug, which would enter a host from the ear and take over complete control of the host, having access to the hosts memories and personality, and being able to masquerade as the host in every way.
I did not start watching Stargate SG1 until 2007, where I immediately noticed a similarity between the Goa’uld and the Yeerks. The Goa’uld were also a parasitic type of slug, which would enter a host and take control while having full access to the hosts personalities and memories.
There are some important differences between the two aliens. The Yeerks can only survive in a host for 3 days at a time, after which they must go to a special pool where the host body is temporarily imprisoned, and the Yeerk will reengergize by absorbing Kadrona rays in a special pool. A Goa’uld by comparison has no such need to leave a host body once in place, and can survive as long as the host in nourished. Appearance wise, a yeerk is described as being a small gray slug about six inches long, which flattens out and encompasses a host brain. A Goa’uld instead seems to attack mainly to the spine, and specific parts of the brain, and resembles a type of snake more than a slug.
A Goa’uld also will heal faster, and make the host stronger than it otherwise would have been. They also have the ability to make the hosts eyes glow while speaking in an unnatural voice the host would be unable to speak in otherwise. It is also important to note that the Goa’uld were shown to only be able to take control of humanoid aliens, while Yeerks were able to take control of anything that had a single large enough brain.
There is also an interesting para rel between the Yoort and the Tok’ra. In SG1, the Tok’ra are a separate faction/species from the Goa’uld, who have evolved/chosen to be symbiotic rather than parasitic. They only take hosts who are willing to take them, taking advantage of having a constant companion and access to a lifetime of knowledge. Similarly, the Yoort in Animorphs were a separate faction or subspecies of the Yeerks who had evolved to be symbiotic with another species, the Isk, to the point where one can not live without the other.
Animorphs premiered in June 1996 while Stargate SG1 followed a year later, premiering in July 1997. The similarities and timing of both of these types of aliens seems like it could be more than just coincidental. The Goa’uld were not mentioned, referenced or even implied in the movie, and were created purely for the spinoff TV show. There seems to be more differences than similarities between Yeerks and the Goa’uld, although I can not help wondering if the creators of SG1 drew their inspiration from Animorphs. All the more likely, there is a far more famous and common precedent in literature, and I am just not aware of it.