Urolithins are microbial metabolites produced from ellagic acid after the intake of dietary ellagitannins by different animals. Urolithin metabolites have distinct UV spectra that enable their detection and differentiation by HPLC coupled with UV photodiode array detectors. Correlations between structural characteristics, including conjugation, with the UV spectra and retention times are established. The production of urolithin derivatives in different animals feeding on ellagitannins, including rodents (rats and mice), humans, pigs, squirrels, beavers, sheep, bull calves, birds, and insects, was investigated. All mammals produced urolithins, and their glucuronyl and sulfate conjugates were the main metabolites detected in plasma and urine. Unconjugated urolithins were detected in feces, ruminal content, and beaver castoreum. Different urolithin hydroxylation patterns were observed for different animal species, suggesting that the microbiota responsible for the metabolism of ellagitannins in each animal species produces dehydroxylases for the removal of specific hydroxyls from the ellagic acid residue. Metabolites were characterized using HR HPLC-TOF-MS and ion trap MS/MS. Insects and birds feeding on ellagitannin-containing foods did not produce urolithins, although they released ellagic acid. Beavers and pigs were able to produce dehydroxyellagic acid derivatives (nasutin A), showing that in some cases the removal of hydroxyl groups from the ellagic acid nucleus can be carried out before the lactone ring is opened to produce urolithins.