Antoninus and Aelius
Titus Aurelius Antoninus and Titus Aelius Aurelius (149): Roman princes, sons of Marcus Aurelius.
Titus Aurelius Antoninus ("Antoninus") and his twin brother Titus Aelius Aurelius ("Aelius") were the second and third child of Marcus Aurelius, the heir apparent of the emperor Antoninus Pius, and Faustina II. The couple already had a daughter Domitia Faustina.
The birth of the two boys was commemorated on coins with the legend temporum felicitas ("happiness of these times"), because their birth meant that dynastic continuity was guaranteed. In ancient Rome, that was another way of saying that a civil war was averted. However, Aelius died within a year, and Antoninus appears to have died soon after. The evidence is, again, coinage: there is a coin that shows Domitia and Antoninus; the next coin shows Domitia alone. There is nothing exceptional to this: infant mortality was high in pre-industrial societies. Aelius and Antoninus were buried in the Mausoleum of Hadrian.
Antoninus must have been the elder of the two, because Marcus Aurelius gave this name to the son who he believed would be his successor. His next son was called Titus Aelius Antoninus (born and died in 152); of the twins born in 162, the elder was called Titus Aurelius Fulvius Antoninus, the younger Lucius Aurelius Commodus.
According to the Greek author Herodian, Marcus' first intended successor was surnamed Verissimus ("most true one").note[Herodian, History of the Roman Empire,1.2.1.] This may refer to Titus Aurelius Antoninus, but - more plausibly - to Commodus' elder brother.
In the end, Commodus succeeded his father, and immediately accepted the name Antoninus. Several later rulers used the name as well.