Rabah bar Shmuel explains that the case of Kil'ayim in the Mishnah is a goat born to a ewe, where the father was a "Seh." The Gemara objects that this is not called Kil'ayim; it is called Nidmeh!
The Gemara clearly assumes that the word 'Seh' is referring to a sheep (i.e. a ram). But we know that a goat is also called "Seh"! Why didn't the Gemara simply explain that the father was indeed a ram, and that is why the child is called Kil'ayim?
Pesach Feldman
Possibly this question can be answered with the help of the Mishnah in Maseches Parah 1:3 which states that whenever the word "Keves" is mentioned in the Torah it refers to a sheep between 0-1 years old, and whenever the word "Ayil" is mentioned it refers to a sheep between 1-2 years old. Therefore, the Gemara here did not want to use either the word Keves or Ayil because this might have implied an age limit. Consequently, it used the word "Seh," which is a more general term. Even though Seh can also mean a goat, nevertheless Seh usually means a sheep. (If the Beraisa would have wanted to say that the father is a goat, it would have used the word "Tayish.")
Dovid Bloom