What is an Ayal?
An Ayal is a hart (Artsroll) or a gazelle (The Living Torah).
What is the comparison to Tzvi ve'Ayal coming to teach us?
Rashi and Ramban: It teaches us that one is permitted to eat Chulin be'Tum'ah. 1
Seforno: It teaches us that one is not required to eat Chulin in a holy location.
Chulin 28a: Since we do not learn anything from Tzi ve'Ayal, the Torah must be comparing Tzi ve'Ayal to Pesulei ha'Mukdashin - to teach us that, like Pesulei haa'Mukdashin, Tzi ve'Ayal require Shechitah. 2
What is the word "Ach" coming to preclude?
Rashi: It precludes an Beheimah of Chulin from the concession of Cheilev - which applies exclusively to Chayos (like to a deer and a gazelle).
Ramban: It precludes the need to offer the Cheilev and the blood of Chulin on a Bamah, and to permit Shechting them be'Tum'ah.
Seforno: It precludes the need to eat Pesulei ha'Mukdashin in a Makom Kadosh - like Kodshim.
Ba'al ha'Tuim: It precludes Pesulei ha'Mukdashin from the Din of Matanos.