The Chimeras were genetically engineered servants of the Goa'uld.
They were created by Nirrti under the orders of the System Lord Ra, with the express purpose of serving as guardians for his daughters. This was to prevent these guards from being mind-controlled by the seductive nature of female Goa'uld. They appeared as animal-headed beings and had a long life span but were infertile, thus making them incapable of breeding. While they might have appeared Human, they were far from it and were genetically engineered creations.
Legends of the Chimeras were known to the Jaffa and it was known that most died out by the middle part of the 18th century. Nirrti attempted to make more such creatures but only succeeded in making abominations.
In an alternate timeline, where Ra discovered Earth earlier due to the accidental alteration of the timeline by SG-1, he sent his daughter Qetesh with her First Prime Cameron Mitchell to conquer the planet. Parts of the invasion forces included several Chimera, who were mistaken for helmeted Jaffa but were actually real creatures. (SG1: "Roswell")