The yellow-tailed woolly monkey, Lagothrix flavicauda (Humboldt, 1812), is a large atelid endemic to the cloud forests of Peru. The identity of this species was uncertain for at least 150 years, since its original description in 1812 without a voucher specimen. Additionally, the absence of expeditions to the remote Peruvian cloud forests made it impossible to collect material that would help to confirm the true identity of L. flavicauda during the 19th and first half of the 20th century. Until now, the specimens of L. flavicauda collected by H. Watkins, in 1925, in La Lejía (Amazonas, Peru) were thought to be the oldest ones deposited in any scientific collection. Nevertheless, after reviewing the databases of the several international museums and literature, we found one specimen of L. flavicauda deposited at the Muséum National d’histoire Naturelle (Paris, France) collected in 1900 by G.A. Baër, in the most eastern part of San Martín (Peru), where the presence of this species was not confirmed until 2011. Thus, Baër’s specimen represents the oldest known specimen of the yellow-tailed woolly monkey and the only one coming from the eastern part of the species’ distribution. Finally, we highlight the importance of online scientific databases for easily diagnosable species. However, caution needs to be taken when using them. We also discuss the value of scientific collections as sources of new discoveries.