Who created titles like "Beethoven's 5th", Beethoven or his followers?

Well, Beethoven’s 5th is literally called “Symphony No. 5 in C minor, op. 67,” but lots of composers wrote 5 or more symphony, so I would say “Beethoven’s 5th” was just an easy way to remember that particular piece without saying the entire title.

But I would like to know when exactly that phrase came into widespread use. I don’t know whether Beethoven’s contemporary audiences would call that particular piece “Beethoven’s 5th” or something else.

His followers. The Sixth Symphony was actually played before the Fifth was.

Neither.

“This is a big subject for the musicologists (and helps keep them off the welfare benefits queue) since they are largely responsible for some of the latest developments in this field.

Basically, Opus numbers are applied by the composer / publisher / musicologist to a work or group of works published / made available at the same time.

The No.1 etc refers to the number of the piece within that opus. So Beethoven’s Opus 27 comprises 2 sonatas Op 27, No 1 in E flat, No 2 in C# minor.

It gets slightly confusing with some works (like those sonatas) because people also have a habit of numbering sonatas/nocturnes etc in sequence, regardless of the opus numbers. So the sonatas I quoted are sometimes called the Sonata 13 in E-flat and the Sonata 14 in C# minor (moonlight). But properly, these are Op27 Nos 1 and 2.” Source

“Beethoven’s 5th Symphony in C minor, op. 67 is rightly considered a natural continuation of Symphony No. 3, &quotEroica”, because it approaches the same themes and it expresses the relationship between particular and general. The name under which it sometimes circulated, ” The Symphony of Destiny ”, is linked to the words of Anton Felix Schindler, his biographer, who, invoking an explanation given by the composer referring to the first bars in Part I of the fifth symphony, stated: ” So pocht das Schicksal an die Pforte! ” (That�s how destiny knocks on your door).”

Source

I would think his followers did.

Well, the first “Beethoven” was named by the late, great, John Hughes. After that, various no-name writers came on to write the sequels, but by that point the whole series really went downhill, IMHO. All of the films after “Beethoven’s 2nd” were direct-to-video releases, which is really a death wish for video sales.