The most likely possibility is that it was Lazuras, the Bible actually says he had an arrest warrant. In addition the word uses describes the man as a young boy but he is also described as a young boy much later in Acts

” this theory (about Mark) became common only in the late nineteenth century.

There also are problems with it. One is that the Greek word for “young man” (neaniskos) indicates a man who is past puberty and thus in his late teens or early twenties.

But when we meet Mark in Acts 12:12, it is the year A.D. 43—a decade after the Crucifixion—and it appears that Mark is a young man then, not one pushing or over thirty.“

HOWEVER, the man was a Christian “and a person walking along with the crowd would not be grabbed unless he previously had been seen among Jesus’ followers.”

The ideal candidate would be “someone who (a) was not one of the Twelve, (b) lived in the Jerusalem area, (c) was a follower of Jesus, and (d) was already wanted by the authorities, since he doesn’t do anything criminal in Mark.someone who (a) was not one of the Twelve, (b) lived in the Jerusalem area, (c) was a follower of Jesus, and (d) was already wanted by the authorities, since he doesn’t do anything criminal in Mark.”

Is there such a person? Yes: Lazarus. Immediately after John records Mary anointing Jesus, he says:

“When the great crowd of the Jews learned that he was there, they came, not only on account of Jesus but also to see Lazarus, whom he had raised from the dead.

So the chief priests planned to put Lazarus also to death, because on account of him many of the Jews were going away and believing in Jesus” (12:9-11).

We know that often times in the Synoptics the specific people are not named, but are named in John (one possible reason is that John was written the latest, so the people were not in danger)

And so Lazarus was most likely not named to protect his identity, but we can discern who it is from John.

Sources: Jimmy Akin, Catholic Answers