The Gemara asks: What is the case of Kil'ayim?
It answers: It is a goat born to a ewe. The father was a Seh.
The Gemara then objecs: This is not Kil'ayim. It is Nidmeh!
The Gemara assumes that "Seh" refers to a ram. Also a goat is called a Seh! Why didn't the Gemara explicitly say that the father is a ram?
Pesach Feldman, Yerushalayim
Perhaps 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 and 1 years old, and whenever the word "Ayil" is mentioned, it refers to a sheep between 1 and 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. Instead it used the word "Seh" which is a more general term. Even though "Seh" also can mean a goat, it usually means a sheep. (Had the Beraisa wanted to say that the father is a goat, it would have used the word "Tayish.")
Kol Tuv,
Dovid Bloom
Thank you for a nice answer.
One small comment - I assumed that the Mishnah says "Ayil - Ben Shtayim" to teach that it must be at least in its second year (at least one month in, if a Palgas does not count as an Ayil). But it does not exclude a sheep that is more than two years old.
Pesach feldman
The Rambam (Hilchos Ma'aseh ha'Korbanos 1:11) writes that the offering from the flock is acceptable only up to exactly two years. If it is more than this, it is considered old and may not be offered. The Tiferes Yisrael (Parah 1:28) writes that even if it is one hour over the limit it is Pasul. He says that the source that hours invalidate for Kodashim is Zevachim 25b.
Kol Tuv,
Dovid Bloom