He served under Pompey and fought at the Battle of Pharsalus, though he fled the battlefield; subsequently pardoned and became a tax collector.

From Wikipedia:

Titus Flavius Petro (fl 1st century BC) was the paternal grandfather of the Roman Emperor Vespasian.

He was a son of a contracted laborer, who each summer crossed the Po to assist the Sabines with their harvests.

Petro was born and raised in Reate, in Sabinia, Italy. He fought for Pompey in Caesar's Civil War and as a Centurion or a volunteer reservist. Leaving the battlefield of Pharsalus in Greece, he secured a discharge with a full pardon and became a tax collector.

He married a woman called Tertulla (c. 40 BC - aft. 9 AD), who was the Preceptor of her grandson Vespasian, daughter of Tertullus and wife, and had a son Titus Flavius Sabinus.[1]

References

• Christian Settipani. Continuité gentilice et continuité sénatoriale dans les familles sénatoriales romaines à l'époque impériale, 2000

1. Suetonius, The Twelve Caesars - Vespasian

Additional Sources

• D. C. O'Driscoll, Flavius

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Titus Flavius Petro

Titus Flavius Petro (fl 1st century BC) was the paternal grandfather of the Roman Emperor Vespasian.

He was allegedly the son of a contracted laborer, who each summer crossed the Po to assist the Sabines with their harvests. However, Suetonius wrote that his careful research failed to support this story. Petro was born and raised in Reate, in Sabinia, Italy. He fought for Pompey in Caesar's Civil War as a centurion or a volunteer reservist. Leaving the battlefield of Pharsalus in Greece, he secured a discharge with a full pardon and became a tax collector.

He married a woman called Tertulla (c. 40 BC - aft. 9 AD), who was the Preceptor of her grandson Vespasian, daughter of Tertullus and wife, and had a son Titus Flavius Sabinus.

Source :

https://en.wikipedia.org/wiki/Titus_Flavius_Petro