Only one beefalo?

Solution 1:

It turns out that you just have to guard that beefalo. You can build a fence around it if you wish. At some point, the lone beefalo will magically create a new baby beefalo. I'm not sure how many days this takes. From this Reddit post the thread poster replied to his own comment with a screenshot of their new baby beefalo. It didn't happen until day 39.

If you're ultra impatient, you can also use console commands.

DebugSpawn("beefalo") - this will spawn one beefalo at your mouse pointer.

c_spawn("beefalo", amount) - adding an integer for amount will spawn "amount" of beefalo.