Our main source for the claim that Jesus was fathered by Panthera is from Origen's Contra Celsum, a refutation of Celsus' Alēthēs logos (The True Word) which is lost. Alēthēs logos was an attack on Christianity by a Greek, pagan philosopher so we can't take it at face-value (just as we don't take Christian apologist literature at face-value).

Celsus wasn't the original proponent of the idea though; the Panthera story is part of a Jewish countertradition found in early rabbinic literature. Christianity, being an offshoot of Judaism, was something that many Jews felt obligated to oppose and so countertraditions that contradicted Christian traditions developed. These rabbinic sources refer to Jesus as Jesus 'ben Pantera' (son of Pantera) or 'ben Stada' (son of Stada). As Stephen J. Shoemaker puts it, "according to this Jewish countertradition, the name ben Pantera referred to Jesus' biological father, who was not Mary's husband, while ben Stada appears to have two explanations: it refers either to Mary's husband, Stada, who actually was not the father of Jesus, or it refers to Mary herself, who was "unfaithful," stath da in the Aramaic, to this husband." Basically, there was a serious attempt to paint Mary as unfaithful, or even, in some cases, a prostitute.

Joseph Pandera shows up in 5th century Jewish traditions that circulated around the Byzantine and Persian empires. Effectively the same story: Mary is seduced, is unfaithful and has a child who learns sorcery in Egypt and who comes back to Galilee proclaiming himself the son of God.

These stories were basically opposing the virgin birth story, and taking it one step further, to counter Christian claims of Jesus' divinity.

The question of whether historians think Jesus was fathered by someone with the name Pantera runs into the same problems as other 'historical Jesus' questions. There simply isn't the evidence to make a case for it. Christian tradition says one thing, Jewish tradition says another, neither are verifiable from a modern standpoint. One claims supernatural occurrence, the other does everything it can to discredit the people involved, and so neither are particularly reliable sources.

I've always held the default stance that Jesus probably existed, but have recently been swayed by the 'consistency of the claim', that is to say the better 'model describing the written evidence we have' that he probably never did. If interested start reading from the following I think credible authors, Richard Carrier, Earl Doherty & Robert Price.