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). Ouch. That was way harsh, Orwell.
Like Boxer, she's loyal and strong—the XX version of Boxer's stereotypical peasant dude. (Check out Boxer's "Character Analysis" to read our thoughts on Boxer as a symbol for the Russian working class.) But Clover is a wee bit more savvy than Boxer. (Could Orwell making a general comment about gender? Hmm.)
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 goat reads her the commandment, "No animal may sleep in a bed with sheets" (6.13). But Clover is actually right—the commandment did say that animals couldn't sleep in beds. It was modified to "a bed with sheets" later. The problem is that Clover isn't equipped to fight back against the pigs.
Check out this key moment 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 is, "If she could have spoken her thoughts." But she can't. She can't read, and she's not so good at using her words. Her thoughts are fuzzy; her language is worse. Clover may sense the hypocrisy, but without the language to express it, she has no choice but to go along with the pigs—and watch Boxer be sent off to the glue factor. Poor Clover.
Clover (a horse) Timeline