[…] thy poor, earth-born companion, An' fellow-mortal! (11-12)
The speaker claims that he has a lot in common with the mouse, after all—they're both poor, they're both inhabitants of the earth, and they're both mortal.
Till crash! the cruel coulter past Out thro' thy cell. (29-30)
The speaker personifies his plough when he calls it cruel. Again, the speaker emphasizes that the plough, a farming tool used to tame the wilderness and cultivate fields, is used to threaten or to hurt the mouse—and, by extension, all of nature.