Mitochondria are of course endosymbionts of the eukaryotic cell — and would appear to be most closely related to rickettsia, the bacteria responsible for typhoid: (realclimate.org)
The expansion of gene families related to immune recognition, endocytosis and caspase - mediated apoptosis indicates the mussel's adaptation to the presence of chemoautotrophic endosymbionts in its gills. (sciencedaily.com)
Plants and various groups of algae also have plastids with their own DNA and, like animals, are accepted to have developed from endosymbionts, in this case cyanobacteria. (fossilmuseum.net)