Two Ariels of Moab
And Benaiah the son of Jehoiada was a valiant man of Kabzeel, a doer of great deeds. He struck down two ariels of Moab. He also went down and struck down a lion in a pit on a day when snow had fallen. - 2 Samuel 23:20 (ESV)
Then Benaiah the son of Jehoiada, the son of a valiant man of Kabzeel, who had done great deeds, killed the two sons of Ariel of Moab. He also went down and killed a lion in the middle of a pit on a snowy day. - 2 Samuel 23:20 (NASB)
And Benaiah the son of Jehoiada, the son of a valiant man, of Kabzeel, who had done many acts, he slew two lionlike men of Moab: he went down also and slew a lion in the midst of a pit in time of snow: - 2 Samuel 23:20 (KJV)
And Benaiah the son of Jehoiada the son of a valiant man, which had done many acts, and was of Kabzeel, slew two strong men of Moab: he went down also, and slew a lion in the midst of a pit in the time of snow. - 2 Samuel 23:20 (1599 Geneva Bible)*
And Benaiah, the son of Jehoiada, the strongest man of great works, of Kabzeel, he smote [the] two lions of Moab, that is, two knights hardy as lions/two strong hardy knights; and he went down, and smote a lion in the middle (of a) cistern in the days of snow. - 2 Samuel 23:20 (Wycliffe Bible)
Benaiah son of Jehoiada from Kabzeel was a vigorous man who accomplished a great deal. He once killed two lion cubs in Moab. Another time, on a snowy day, he climbed down into a pit and killed a lion. - 2 Samuel 23:20-21 (The Message)
Meaning
The meaning is unknown. You can see versions have tried translating it as a given name, as men as strong as lions, as lionlike men, as lions, and simply as "ariel."
The Hebrew word for "lion" is aryeh. The "-el" implies a religious dimension that transcends mere lions.
Brian Godawa, a screenwriter and author, suggests these are were-lions. I don't know what the basis for that is, but I'm including it anyway.