There is a very productive suffix in English coming from Latin: '-or': doctor, actor, aviator, etc. meaning 'the person that does the thing'.
It is spelled '-or' but is pronounced to rhyme with the English Native spelling of the similar meaning '-er' or in IPA for American English as from the NURSE lexical set: /ər/ or /ɚ/ and in non-rhotic BrE /əː/.
Pretty much all words ending in '-tor' are pronounced this way.
- /'dɑk təɹ/
- /'æk təɹ/
- /in 'ven təɹ/
Except for 'mentor'.
- /'men ɔɹ/
It is pronounced with the 'NORTH' vowel: AmE /ɔɹ/ and BrE /oː/.
There doesn't seem to be any logical (similar history) or phonetic (rule based) reason for this. From an automated search of words, only 'guarantor', 'or', 'nor', and 'tor' had the same final syllable pronunciation, but none sharing the same stress pattern with 'mentor' (and the last three are not the suffix anyway). And 'inventor', which is very close except for the first syllable, does not share the last syllable.
Can anyone throw any light on this? Was 'mentor' imported or created special? Is it a 'spelling' or 'faux-highbrow' pronunciation like sometimes 'actor' or 'realtor' might be pronounced? Or is it just an anomaly as they happen sometimes?
Note: for pronunciation reference I used the CMU Pronouncing dictionary for automating the pronunciation search and facilitating making general categorical statements like 'There are no other words like...' . It has only one pronunciation for each word (i.e. no variants), and only AmE (so I'm unsure about some of the BrE versions).