Clover is Boxer’s companion, a “stout motherly mare approaching middle life, who had never quite got her figure back after her fourth foal” (1.3). She is a good-natured horse, just as loyal and strong of spirit as Boxer, even if she doesn’t possess the same physical strength. She continually warns him not to over-exert himself, though Boxer never listens to her.
Though Clover is an extremely loyal horse, it seems that she has slightly more capacity for doubt than her companion. When the pigs begin sleeping in beds, she thinks that she remembers a rule against animals sleeping in beds. Since Clover can’t read, Muriel the horse reads her the (modified) commandment, “No animal may sleep in a bed with sheets” (6.13). Clover is obviously right to be suspicious. The only problem is that she doesn’t have enough faith in her memory or herself; the result is that she has no choice but to be gullible and convinced by the pigs.
We get the deepest insight into Clover’s character after Napoleon begins executing other animals:
As Clover looked down the hillside her eyes filled with tears. If she could have spoken her thoughts, it would have been to say that this was not what they had aimed at when they had set themselves years ago to work for the overthrow of the human race. These scenes of terror and slaughter were not what they had looked forward to on that night when old Major first stirred them to rebellion. (7.30)
The key phrase here is, perhaps, the clause, “If she could have spoken her thoughts...” The narrator reminds us that Clover can't read and isn't able to express herself well in words. So her doubts are fuzzy and she can’t even put into words what has gone wrong on Animal Farm. Clover is deeply bothered by the violence, and she senses hypocrisy, but without the words to define what is going on, she has no choice but to go along with the pigs.
As Boxer ages, he is determined to re-build the windmill, and Clover carefully reminds him that “A horse’s lungs do not last for ever” (9.1). When his lung collapses, she stays with him and tends to him, but again, she is not quite suspicious enough of the pigs’ intentions. It takes Benjamin to explain to her that Boxer is being sent to the knackers (a man who buys worn out farm animals and slaughters them). Clover tries to warn Boxer, but by the time she does it is too late.
Like Boxer, Clover is probably an allusion to the Russian proletariat – the working class. She takes care of her hard-working companion, and though she is concerned for his well being, she cannot protect it because she doesn’t realize just how cruel some animals can be.