Think of everything good about the rabbits in this book, especially Hazel—now take them all away and you've got Cowslip and his rabbits. Cowslip might as well wear a sign around his neck saying "a deal that looks too good to be true may not be true." Or maybe his sign should be "if there's a problem, you should deal with it rather than ignore it." Or how about this sign: "Don't let people eat your friends"? There are a lot of lessons that Cowslip offers. But they all boil down to "Don't be Cowslip."
When we first see Cowslip, he gets described with two qualities: he's rich looking, good-looking and a little weird. He's "sleek," with claws and teeth "in perfect condition"—but he also has "an unusual smell" and has "unnatural gentleness" (12.34-6). One of his facial expressions is "his unnatural smile," which is a nice mix of positive (yay smiles) and weird (boo unnatural things) (14.48). Frankly, he sounds like a secret serial killer.
And though Cowslip isn't the Chief Rabbit of the warren, he is a very good example of the warren. The other rabbits are almost all described as well fed and weird, just like Cowslip. And not in a fun, arty, dyed hair sort of way. They're weird in the "are they planning to kill us?" sort of way, never answering questions and whispering about other rabbits.
We've got a section in "Character Roles" on Cowslip as a foil for Hazel, because that's really his most important role here. He shows how good Hazel is as a leader by being so bad himself. When faced with a problem, Cowslip decides to be pretty passive and just let it happen. ("It" in that sentence means that he lets the farmer catch, kill, and eat his rabbit friends.) In contrast to Hazel, who tries to help rabbits out of traps, Cowslip invites Hazel's group into a trap.
If Cowslip is the anti-Hazel, then Silverweed is both anti-Dandelion and anti-Fiver. He's the young rabbit who gets to tell stories in Cowslip's warren. Except, instead of storytelling like Dandelion, he recites poems. And instead of celebrating the cleverness that rabbits need, Silverweed thinks that "Rabbits need dignity and, above all, the will to accept their fate" (16.12). In other words, Silverweed celebrates passivity, in all its creepy glory. Maybe that explains why Cowslip is so passive in the face of danger—because the ideal held up in his culture is resignation to death.
But these aren't just poems that Silverweed says. They're nervous trances, very like the trances that Fiver goes into. Here's Silverweed going into a trance:
He had a wild, desperate air and his ears twitched continually. As he began to speak, he seemed to grow less and less aware of his audience and continually turned his head, as though listening to some sound, audible only to himself, from the entrance tunnel behind him. (16.21)
If you squint, that looks a lot like Fiver, who is known for his nervous twitchiness and trance-like states. So Silverweed may have some of the same psychic, mystical powers that Fiver has, but his reaction is totally different: When Fiver sees a field of blood, he runs away. When Silverweed sees a field of blood, he just says "oh well, nothing to be done and time to accept my fate."