So far as I have been able to discover the full name of the brother of the Emperor Claudius and father of Caligula was
Nero Claudius Drusus with the added honorific of Germanicus. This appears to be his father's full name as well (indeed the honorific was awarded to commemorate his deeds rather than those of the son).

My question is, where does this leave the praenomen? It would appear to be Nero but I thought that had always been a cognomen of one of the branches of the Claudii. Was the name Nero promoted to become an acceptable praenomen or did these two gentlemen have another praenomen and, if so, what?

Can anybody help?

Volusian Amenemhat