Commentary for Avodah Zarah 48:17

R. Yitzchak Nafha’s version of the song is not Scripture, but rather a song composed by the rabbis (or someone else) themselves. The acacia is a reference to the ark, which was made of acacia wood.
R Ashi teaches R. Yitzchak’s song in another context—it refers to the song that the Israelites sang when they wandered around in the wilderness with the Ark. Maybe R. Ashi did not like the idea of singing cows.