I have watched the TV Series Bewitched several times and each time we come to someone asking Endora's last name, she always says 'You could never pronounce it'. I haven't watched the supplementary materials though, so does anyone know what Endora's surname was?
Within the TV series, almost none of the witches have surnames. This includes Samantha and her parents; Maurice and Endora.
Out of canon, in the 1965 authorised novelisation by Al Hine, Samantha's maiden name is revealed to be "Dobson" however this appears to have been based on an unused shooting script that references her father as being "John Dobson, burned at the stake in York in 1538".
This was later retconned with the introduction of Maurice.
Beginning of Season 5 Episode 18 (Samantha, The Bard)
Samantha seems to rhyme her last name: