That’s a good question! I guess it depends what you mean by food chain – or rather, which food chain you’re talking about. For example, some ocean food chains will end in things like the great white shark, for which there is no other natural predator (by which I mean, there isn’t anything in it’s environment that relies on the great white as a main food source). So in that sense, there are lots of animals that are top of their respective food chains.
Prior to the existence of humans, there were animals roaming the earth that probably had very few natural predators – for instance, large carnivorous dinosaurs like T.Rex would have been at the top of their food chains. But I suppose that you could make the argument that things like certain types of bacteria or virus are actually at the top of the food chain!