1) It does not matter that the verse of "Shor O Kesev O Ez Ki Yivaled" is talking about Korban, and we do not require a Gemara that connects these Halachos, because all we learn from the Pasuk is that the definition of a Shor is an animal that is born. This is a "Giluy Milta" (see Bava Kama 2b, "Giluy Milta b'Alma Hi"; that is, the verse merely reveals that the word "Shor" means an animal that was born in a natural way).

2) Yes, indeed according to this a Yotzei Dofen also cannot be tried as a Shor ha'Niskal because it is not an animal that was born naturally.

3) For further support: I saw that Rashi in Chulin (74b, DH d'Gamar) writes that Ben Peku'ah is excluded as a Korban from the verse of "Ki Yivaled," which excludes a Yotzei Dofen.

We learn from Rashi that a Ben Peku'ah and Yotzei Dofen are equated with regard to the verse of "Ki Yivaled." We now may take this further and say that this does applies not only to Korbanos but also to anything derived from the verse of "Ki Yivaled," and thus Ben Peku'ah and Yotzei Dofen will always be equal.

Therefore, since the Rashba writes that only a Shor born naturally is considered a Shor for Shor ha'Niskal purposes and, consequently, Ben Peku'ah is excluded, it follows that Yotzei Dofen is also excluded from Shor ha'Niskal.

4) I later found a passage in the Imrei Binah (by Rav Meir Auerbach zt'l; Yoreh Deah, Dinim of Shechitah #13, DH v'Da deY'I) which gives us a very different understanding of the Rashba.

The argument is based on the Mechilta (printed in the Malbim Chumash) on Shemos 21:28, "And when a Shor kills a man or woman he must be stoned." The Mechilta comments on this, "I know only that if a Shor killed a person it must be stoned. How do I know that any animal that kills must also be killed?"

5) The Mechilta answers that we learn from the word "'Shor' stated at Sinai." (It appears that this means that in the Aseres ha'Dibros, in Shemos 20:14, we are commanded not to covet the "Shor" of our fellow man. This does not refer only to a Shor, but to every animal. Similarly, in the second set of the Aseres ha'Dibros, in Devarim 5:14, we are commanded not to work with our Shor on Shabbos, and this prohibition includes all animals, not only Shor.)

We learn from the Mechilta that all animals, not merely Shor, are liable for Sekilah if they kill a person.

6) According to this, the reason why all animals who kill a human are killed is that they are included in the Din of Shor ha'Niskal; not merely because they are a public menace, as you write, Meir.

7) The Imrei Binah concludes from the Mechilta that if all animals who kill are included in the Din of Shor ha'Niskal, then there is no reason why Ben Peku'ah should be any different.

8) So how does this fit with what the Rashba writes, that a Shor is called a Shor only if it was born naturally? The Imrei Binah explains that the Rashba is referring merely to the "Pashta d'Kra" -- the simple meaning of the verse. The simple meaning of the word "Shor" in Shemos 21:28 is a Shor that was born naturally. The Mechilta makes a "Ribuy" to include the death penalty even for an animal which is not a Shor.

9) However, in the second half of the verse in Shemos 21:28 ("Sakol Yisakel ha'Shor"), the word "Shor" refers to the simple meaning of "Shor" without the Ribuy. This, according to the Rashba, is only a Shor that was born naturally. The word "Shor" here cannot refer to Ben Peku'ah. This, then, is how the Rashba answers the question of Tosfos, that the verse is required for Ben Peku'ah.

10) It would appear from the above that according to the Rashba either a Ben Peku'ah or Yotzei Dofen Shor that killed would be stoned as a Shor ha'Niskal (not merely as a public hazard) because there is a Ribuy that all animals that kill people must be killed.

Dovid Bloom