response = """
<tr class="someClass"><td></td><td>chicken</td></tr>
<tr class="someClass"><td></td><td>chicken</td></tr>
<tr class="someClass"><td></td><td>cow</td></tr>
<tr class="someClass"><td></td><td>cow</td></tr>
<tr class="someClass"><td></td><td>cow</td></tr>
"""
lines = response.split('\n')
cows = []
for line in lines:
    if 'cow' in line:
        cows.append(line)
print(cows)
Complex non-greedy matching with regular expressions