Selenosulfide groups can be found in almost all living organisms as part of various peroxidase enzymes, such as glutathione peroxidase and thioredoxin reductase. They are formed by the oxidative coupling of  selenocysteine and cysteine residues. This reaction is powered by the decomposition of cellular peroxides, which can be highly damaging and a source of oxidative stress. Selenocysteine has a lower reduction potential than cysteine, making it very suitable for proteins that are involved in antioxidant activity.
Selenosulfides have been identified in some species of Allium and in roasted coffee.  The mammalian version of the protein thioredoxin reductase contains a selenocysteine residue which forms a thioselenide (analogous to a disulfide) upon oxidation.