Whenever a movie is extremely successful, people start coming out of the woodworks filing lawsuits and trying to cash in. This happened with Disney's Frozen when filmmakers Kelly Wilson and Neil Wrischnik filed a lawsuit claiming that the studio stole the concept from their short film "The Snowman." You can read the details about that here.
The short is somewhat similar to the teaser trailer that Disney released for the film featuring the snowman Olaf and the reindeer Sven as they go out on a frozen lake trying to get Olaf's carrot nose. The same thing happens in the short film, only instead of a reindeer there's a group of rabbits.
It's actually a entertaining short, but that doesn't mean Disney stole the idea. Disney had this to say in a statement, "The claim is utterly without merit and we will defend against it vigorously." You can watch both of the shorts below and decide for yourself.