In Watership Down, the Threarah is maybe the second-best Chief Rabbit—after Hazel, of course—and his warren ends up exterminated. Which is a way of showing that it's hard out there for a rabbit. Even when you've got a relatively smart and dedicated rabbit, that doesn't amount to much next to the human habit of pumping poison gas into a warren.
And this is a tragedy because the Threarah is so close to being as good as Hazel. When we first meet him, we get a catalogue of his greatest hits: he saved the warren from disease by exiling sick rabbits during an epidemic (which is bad for those sick rabbits, but good for the warren as a whole), he even once lured a stoat (a weasel-type predator) into human company so some human could kill it. In other words, he's willing to make hard decisions and to risk his own life to save the warren.
So why does the Threarah fail? According to Hazel, the Threarah "doesn't like anything he hasn't thought of for himself" (3.26). That is, unlike Hazel, the Threarah isn't very good at listening to the advice of other rabbits. He's smart and dedicated, but too proud to listen to Fiver when he warns about upcoming danger.
But then again, as we saw, Hazel's got a little pride problem, too. So maybe the Threarah fails because he's too settled—too worried about making a change. In fact, when Holly comes to Watership Down, he reports that the Threarah said something much like that: "Almost always, it's better for the warren as a whole if rabbits sit tight and do their best to dodge their dangers underground" (21.3). So the Threarah is still thinking like a leader about "the warren as a whole"; but his "almost always" is too broad a generalization. Sometimes it's necessary for the rabbits to be active rather than hiding from dangers. ("Active" here means "running away from dangers.") Frankly, that's the kind of thinking we expect from someone named after a tree ("threar" means "rowan tree" [2.22]). After all, trees are notoriously lazy about moving, the slobs.
There are many other rabbits in Sandleford Warren who don't follow Hazel—and we hear most of their names when Holly and Bluebell tell the story of who died. (Answer: most of them.) The only other rabbit who plays a big part in the story is Toadflax, a member of the Owsla and a bully.
We meet him in the first chapter, threatening Hazel and Fiver over some tasty flayrah (treat). It's basically like people threatening someone over lunch money. Toadflax is such a bully that Hazel is surprised when he doesn't bully rabbits first and ask questions later (4.8). For Hazel, Toadflax is a model of what not to be.
We never get to meet Pimpernel while he's alive, but we sure remember how he dies. According to Holly, Pimpernel was a little sick after the whole "humans gassed us and shot us" event. So when they reach Cowslip's warren, Pimpernel is too sick to protect himself from Cowslip's rabbits.
As Holly says, "after all he'd been through, poor Pimpernel was killed by rabbits. What do you think of that?" (21.41). We'll tell you: we think that's lots of irony right there. Sad, sad irony. It really is hard out there for a rabbit.