In the poem ' A River', the cows have been named as Gopi and Brinda. There are a couple of view points on the reason for giving them the names. One can be to give a simple 'light-hearted' touch to the sad and strong poem like this. The whole poem saddens up the mood of a reader but the mention of these two names lightens up the tone.
Another reason can be that cows are considered as sacred animals in India, and also have been associated to the name of Lord Krishna (as he loved drinking milk and eating butter). The named 'Gopi' can be traced back to those days. Also being so sacred animals, cows are given the same status as humans. Giving them names, hence, becomes an essential part of that.