Adam Clarke on John 18:15

And - another disciple - Not That other disciple, for the article is omitted by AD, two others; some editions; Syriac, Persic, Gothic, and Nonnus. So the Vulgate is to be understood. There are many conjectures who this disciple was: Jerome, Chrysostom, Theophylact, Nonnus, Lyra, Erasmus, Piscator, and others, say it was John. It is true John frequently mentions himself in the third person; but then he has always, whom Jesus loved, as in Joh 13:23; Joh 19:26; Joh 21:7, Joh 21:20, except in Joh 19:35, where he has plainly pointed out himself as writer of this Gospel; but, in the place before us, he has mentioned no circumstance by which that disciple may be known to be John. To this may be added that John being not only a Galilean, but a fisherman by trade, it is not likely that he should have been known to the high priest, as it is here said of that disciple who followed Jesus with Peter. See Bishop Pearce and Calmet. The conjecture of Grotius is the most likely: viz. that it was the person at whose house Jesus had supped. St. Augustin, Tract. 113, speaks like a man of sound sense: We should not decide hastily, says he, on a subject concerning which the Scripture is silent.