These are called Avos because they are written in the Torah explicitly, and the Gemara explains the Toldos are.

The order in which they are written in the Parsha they are ordered in the Mishna, for the first Parsha discusses the Ox, the second, Pit...

Not, here is, Bull like the Mave - It is to say, had it been written, in the torah (only) Ox, does not come out Mave from it, and due to this it needed to write it, and for this it took the first part to be like the Mave, and did not take to it like the order, (i.e.) not in this the bull like the pit, more so due to, that it was not going to be able to teach, not this and this that have in them breath of life, that here, pit not in it breath of life, and a reason that is different, that there is here a novelty to teach us, that even though there is in both of them breath of life, does not come out one from its fellow. And the Gemara explains (later) what "not this" (lo harei) that it is said here.

Like the fire, that does not in it have air of life - And if not written by the Torah one would have said to exempt.