So in reference to this question, an ectopic ovary is ovarian tissue that is "misplaced", or found where it is not usually expected to be found. This tissue can be microscopic. Ectopic ovaries are not a common occurrence in veterinary medicine, what is more commonly seen is "ovarian remnant syndrome" where some mircroscopic ovarian tissue has remained after a spay (ovariohysterectomy) and the animal shows signs of heat after surgery.
I once had a case of a young cat presented for a spay who had not yet had a heat cycle yet (due to young age) and no visible ovary could be seen on one side during surgery. I called in my colleague to verify, and we could not locate the ovary for that side. The cat was sent home with the warning that she could come into heat if there was tissue that we just couldn't see. Several months post-operatively, the cat did indeed experience signs of estrus. When exploratory surgery was performed, there was some ovarian tissue, smaller and less distinct than a normal ovary, but in a nearby location to where a "normal" ovary would be found. This tissue was now enlarged enough to see (and remove) due to the influence of the estrus cycle hormones.
Other organs are more commonly "ectopic", such as testicles (in the abdominal cavity or inguinal ring, also called undescended, since testicles do start out in the body cavity and "migrate" into the scrotum at birth or shortly thereafter), ureters (tubes that carry urine from the kidneys to the bladder), and thyroid tissue (can be found anywhere from the larynx to the diaphragm).
Related Reading:
How Long Does the Heat Cycle Last in the Cat?
How Long Does the Heat Cycle Last in the Dog?
My pet was spayed, but now looks as if she is in heat. How can this be?
