Bekhorot 40B

ומעשה נמי שלחי התחתון עודף על העליון, שאל רבן שמעון בן גמליאל לחכמים ואמרו: הרי זה מום.

The Talmud continues this version of the Mishnah: And there was also an incident where the lower jaw of the firstborn protruded beyond the upper jaw, and Rabban Shimon ben Gamliel asked the rabbis for a ruling, and they said: That is a blemish that enables the slaughter of the firstborn.

והא גבי אדם הוא דתנן: שפתו העליונה עודפת על התחתונה, או שתחתונה עודפת על העליונה ⁠— הרי זה מום. גבי אדם הוא דכתיב: ״איש איש מזרע אהרן״, איש ששוה בזרעו של אהרן, אבל בבהמה לא!

The Talmud challenges: But it is with regard to the blemishes of a person that we learned in a Mishnah (44a): If his upper lip protrudes beyond the lower lip or his lower lip protrudes beyond the upper lip, that is a blemish. And it is only with regard to a person that it is written: “Whatever man of the seed of Aaron” (Leviticus 22:4), which teaches that in order to be fit for the Temple service a priest must be a man who is equal to the seed of Aaron, i.e., he has an ordinary appearance like other priests. But this requirement is not stated with regard to an animal, and therefore an animal should not be considered blemished if its lower lip protrudes beyond the upper lip.

אמר רב פפא: לא קשיא, הא דאית בה עצם, הא דלית בה עצם.

Rav Pappa said in response: That is not difficult; this Mishnah is referring to a case where the lower jaw contains a bone that is longer than the upper jaw, in which case it is a blemish even for an animal, whereas that Mishnah is referring to a case where the lower jaw does not contain a bone, and therefore it is a blemish only in the case of a person.