Official uses of the name
Most notably, a pin (depicted here) confirms the name, although merchandise has often been known to be of questionable canonicity.
Host A Ghost
In Disney's line of Host a Ghost Spirit Jars, he is given the name Prof. Phineas Plump
During the limited interactive mystery game, Ghost Post, he is heavily implied to be the ghost yearning to hitchhike named Phineas.
In the mobile-phone-only video game Disney Crossy Roads, his name is given as Prof. Phineas Plump.
- Many characters in Haunted Mansion lore share the first name of Phineas.