It's hard to find instances of it happening. For instance birds evolved from dinosaurs, going from animals with brains the size of peas to fairly intelligent, with some of them even having the ability to use tools and solve fairly complex problems. Even all mammals evolved from marsupials, which if you know anything about them, their low metabolisms render them fairly stupid. Koalas are smooth brained, our wrinkles didn't evolve until we gained placentas.
Perhaps some rodents did actually evolve from animals which were more intelligent, but it's impossible to know for sure.
