any substantive difference

[JeffD]

Any real difference between an electric mandola and an electric tenor guitar?

[sonic]

Simply put probably not.

Depends on scale length, and if the Mandola has coursed strings or single. If you're talking single strings then scale length and tuning are the only differences.
Mandola is usually shorter scale 15" to 20" than a Tenor and tuned CGDA. Tenor guitar is longer 20" to 24" and Tuned GDAE an octave below a mandolin.
There is some overlap of scale lengths to names, add to that the UK/European USA differ slightly in the use of some names for Mandolin family instruments you can end up with differences in name use.

[JeffD]

Yea I was thinking of a four string electric CGDA mandola, and I saw a four string tenor guitar, and I thought, wait, is there a difference? And then we throw electric octave mandolin in the mix.

It can get confusing.

[sonic]

Easiest way is to think of them by scale length and usual tuning to string gauge.
13" to 15" mandolin
15" to 20" Mandola
20" to 25" Octave mandolin, Tenor Guitar.
But its not always that simple as there's some overlap with long/short scale for tuning instruments.
For instance I have a 20" five string electric octave mandolin, at least that's what I'm calling it.
At the end of the day if you can tune it to the notes you want and play it. Then whats's in a name?

[mandroid]

Length of the neck ?

Gibson's TG and EM of same era Shared same model of P90 pickup ..