With names of living things, one is able to distinguish between the name it is called from the day it is born – in which its feminine gender cannot be derived from its masculine gender (like גמל שפן חזיר שור ושה, and ארנבת יונה חסידה, which are said for a male and for a female) - and the adjective specifying whether it is adult, which can be found in feminine gender (like פר פרה, כשב כשבה, לביא לביאה):

The noun כבש (lamb) is in its first year, and after its first year, it is called an איל (hart). And when the sages of blessed memory say that also a newborn is called an איל, this is in the language of man, but in the language of the Torah, it is an adjective denoting power and growth. And moreover the name שעיר (goat).

But צאן (flock) is a species name, and thus it adds ואם מן הצאן..כשבה או שעירה (from the flock, a lamb or a goat)⁵⁸⁶ to teach that one that is deaf or dwarfish is also kosher.

footnotes: ⁵⁸⁶ Leviticus 5:6.