The Gemara opens with the obvious question: how can we possibly understand the suggestion that the cow spoke? Rami bar Hama explains that the cow was lying down and a person who was trying to get the cow to stand up said, “This cow thinks that no one can get her to stand up? I will be a nazir from her meat if she gets up on her own!”
Tosafot and other rishonim point out that it is not uncommon to find people who project thought and speech onto animals – or even inanimate objects. One example that is brought is the passage in Yonah (1:4) that, “The boat thought that it would break apart.” The general approach of the rishonim to our Mishna is that it is describing a colloquial statement made by a person, which is not made with the clarity of meaning and purpose that we ordinarily expect from a halakhic statement.
In his responsa, the Rashba brings an explanation in the name of Rabbeinu Barukh that we view the statement as follows. According to the person’s statement, we view the behavior of the cow as a declaration of a vow of nezirut not to stand. The person follows this by announcing that if the cow “stands” (thereby abiding by her “vow”) then he will accept nezirut. The Rosh offers the approach of Rav Yosef ish Yerushalayim who says that the cow’s behavior appears to be announcing that she is presenting herself as a nezira – that she is forbidden to all. The person’s statement essentially agrees – if he cannot rouse her, her meat will be forbidden to him like a nazir.