To overfeed someone or something is to give them too much food. The object matters: you overfeed a baby, a dog, the fish, farm animals — you are the one holding the food. It is not the same as overeat, which is what you do to yourself. People use it a lot with pets and little children; vets and grandparents argue about it. You can stretch it to plants and plant food, but the core picture is feeding. The past is overfed, like feed / fed. Though actually, kind people often overfeed because they love the animal, which is why the word so often comes as a gentle warning, not a shout.