A number of personal names in the works of Plautus end in -um in the nominative singular, which struck me as odd when I first encountered them, since nominative singular -um is characteristically found at the end of neuter nouns (specifically, those of the second declension). An example is Stephanium. In fact, all of these names seem to end more specifically in -ium.
I found names ending in -ium discussed briefly in one section of "Some Roman Slave-names" by F. F. Bruce. Bruce says that these are from Greek diminutives in -ιον, which were "mainly names of women". Women's names ending in -ium are grammatically feminine in Latin.1 (I'm not sure how names ending in -ιον behave in Greek.) Harm Pinkster gives the phrase "amoena Stephanium" as an example of this principle of semantic gender assignment2.
According to Anna Morpurgo Davies, although some women's names in Greek are neuter, no men's names in Greek are neuter3. Nevertheless, in the context of Latin, there does seem to be at least one example of a male name ending in -ium: Paegnium, the name of a male slave in Plautus' Persa, and one of the names of the character Tyndarus in Plautus' Captivi4. The Lewis and Short entry for Paegnium says that it comes from the Greek neuter noun παίγνιον 'plaything, toy', which LSJ says could also be used of persons in the sense "darling, pet". Philippides suggests a comparison with the Latin term delicium (p. 107).
Are there any examples other than Paegnium of male names ending in -um?
Henry John Roby, Grammar of the Latin Language from Plautus to Suetonius, Part 1, p. 106
"Tyndarus' Past: The Name Paegnium in Plautus' Captivi", by Katerina Philippides, Classica et Mediaevalia 62:1 (2011).