When a woman gives birth to an infant, even if it is shaped like a lamb or a cow, the infant is considered by one of the sages to be a human for all intents and purposes. One sage, R’ Jeremiah, asked what the rule would be if the father of the “lamb” (which is considered a person) married the lamb off to a man. Would that man be allowed to marry the lamb’s sister? (A person is forbidden to marry two sisters in their joint lifetimes.) The scholars asked R’ Jeremiah: This infant will not live, so the question is irrelevant, for after its death the sister would be permitted in any case. One of the sages answered that R’ Jeremiah wished to entertain his colleagues by asking a question which has no practical application. The scholars lent support to the argument which states that a human infant in the form of an animal will not live by quoting another accepted law: If a woman gives birth to a lamb or a cow and then again becomes pregnant and gives birth to an infant in a normal (human) form, the second son is considered the eldest for purposes of inheritance. (He inherits a double portion.) Were the lamb or the cow which was born to the woman to live, the second son would not be considered the eldest, so we see that the lamb or cow is not expected to live. A different sage, Rava, rejected this proof, and argued that the infant shaped like a lamb or a cow would indeed live, but the second son is considered to be the eldest for purposes of inheritance because the role of eldest is determined by the father’s sorrow upon the death of the son. The father would not be sorrowful if an infant in the shape of a lamb or of a cow would die, but he would be sorrowful at the death of the second son, who was formed normally, so the second son is considered the eldest.
(Babylonian Talmud, Tractate Niddah 23a-b)