The use of metal nanoparticles as supplements of animal diets does not always bring unambiguous results. There are many reports in the literature about the multifaceted effects of this type of supplementation on the animal organism. Therefore, the aim of the paper is to present the current knowledge of the possible application of nanometal forms in animal nutrition and its potential benefits and threats. The positive effect of nanoparticles used as feed additives has most frequently been reflected in an increase in body weight, higher average daily gain, or improvement of the FCR value. In some cases, however, the effect of nanoparticle addition to diets was indiscernible. The potent antibacterial activity of nanoparticles, especially against Gram-negative bacteria and Gram-positive bacteria, is regarded as a positive effect. In turn, the probability of their toxicity is a potential risk in application thereof. Supplementation of diets with nanometals has been accompanied by pathological changes in animal tissues, primarily in the pancreas, kidney, liver, rumen, abomasum, small intestine, adrenal glands, and brain. Additionally, at the the cellular level, nanoparticles were found to induce toxicity, inflammatory excitation, and cell death. Oral administration of nanoparticles induced a risk of malfunction of the nervous system and even impairment of cognitive processes in animals. The increasing knowledge of the possible toxic effects of nanoparticles on the animal organism suggests caution in their use in animal production and necessitates further precise investigations in this area.