Ceremorphosis

Ceremorphosis was a bodily change that occurred when an illithid tadpole reached maturity and was inserted into the brain of another humanoid being, usually a human. The tadpole ate away the victim's brain matter and essentially replaced the brain, erasing all of the subject's personality and memory, but leaving the physical body alive and under the tadpole's control. After this, "morphological transformations" occurred and a new illithid was created.

Half-illithid
If this was done with a non-human, the result was technically a "half-illithid". They did, however, still possess most of the powers of a regular illithid. Examples of creatures beyond humans that had undergone ceremorphosis successfully were:
 * Tzakand
 * Mozgriken
 * Mindwitness
 * Uchuulon
 * Urophion