When a fox with a slit throat shows up in Lissa's bed, everyone thinks it's just some nasty prank, but when a second dead animal arrives—this time with a threatening note—Lissa and Rose start to panic. The note reads: "I know what you are. You won't survive being here. I'll make sure of it. Leave now. It's the only way you might live through this" (12.47). Okay, that's super creepy.
Lissa and Rose panic about the dead animals and the note because they're afraid someone knows about what happened with the raven and is going to hurt them. And the thing is, they're partly right. Victor has heard about Lissa's little healing—or should we say resurrection—of the raven, but he's interested for his own selfish reasons, not to harm Lissa. It turns out he's the one whose been sending the dead animals (via his daughter and Lissa's roommie, Natalie) so he can tempt Lissa to heal something again. It's just like a test run before he lets her take a swing at him.
Even though Victor is trying to get to Lissa and her magical healing powers and not actually trying to hurt her, the dead animals signify a big scare for the girls' secrets. They're worried about what will happen if someone—anyone—finds out that Lissa can not only do compulsion and other forbidden magic, but actually heal other people. The dead animals are a warning to them about what could happen if they don't do anything about their little secret, and a reminder that Lissa is pretty darn exceptional.