One may type "pseudonym" into an "Anywhere" box at MathSciNet and find 44 hits. Many of these are not relevant to the question at hand, but I'll post any that I find that haven't been posted here already. Here's one: Christian Tapp, Kardinalitat und Kardinale, MR 2006h:01012, the review by Volker Peckhaus says that in Chapter 5, "We learn about [Georg] Cantor's pseudonyms such as Vincent Regnas, Jorge Vincente Monteador de Montemor, and others...."