Last Names are really hard

If it’s a fantasy culture, think about the reason people have more than one name. Historically (in Europe, at least) it was to distinguish between two people you knew who had the same name. You could make that distinction several different ways

• Job: Bob the tailor and Bob the cooper (barrel-maker) become Bob Taylor and Bob Cooper (Hermione Granger’s last name means ‘farmer’)
• Family: Bob, son of andrew (Our Supreme Lord and Overseer) and Bob, son of John become Bob Anderson and Bob Johnson
  In some places, when a woman married, her husband’s name would become her last name: when Susan marries John she becomes Goody (short for ‘goodwife’, basically an old way of saying ‘Mrs’) John
  European societies were typically patriarchal and the dad/husband’s name was carried on, but in some cultures boys get their dad’s last name and girls get their mother’s.
• Location: if both Bobs are farmers, we can’t distinguish by job, so let’s distinguish by where they live - Bob by the brook and Bob by the forest become Bob Brooks and Bob Forrest
  Nobles typically had the name of their ‘lands’ as well, so the guy in charge of Northcote might be called Lord Northcote.
• Physical characteristics: Bob with the blonde hair and Bob who’s really really tall become Bob White and Bob Long
• Great deeds: in some cultures you get a second name once you’ve proven yourself in some way. Bob who killed a rabid wolf and Bob who steered a ship home through a terrible storm might become Bob Wolfsbane and Bob Stoutheart

If you think about what your fantasy culture valued in the past, this might give you some ideas for how they’d choose to distinguish people from one another, and you can use that to come up with some last names that will fit your fantasy world just right. :)

All my examples are in English, but you could use the same ideas for any language you’re using for your culture, even if it’s one you made up.