The only pleco that I've heard being predatory is the Vampire pleco, as someone mentioned earlier, but I think ultimately all plecos are omnivorous opportunistic scavengers like almost every catfish. Some are also more 'aggressive' and carnivorous like the L25 scarlets, but I just bought my third and I think it's more the case that they have so many dangerous scutes on their body that most fish learn quickly not to invade their space.