The wording of the translation here has brought about some confusion. There is no character named "Ulysses Pidytes." Rather the sentence is describing the deaths of three men on the Trojan side as caused by three men on the Greek side, just as in the broader context of this whole first section of Book 6: Polypoetes kills Astyalus, then "Ulysses" kills Pidytes [of Percote], and then Teucer kills Aretaon. The translator has simply chosen not to repeat the verb in each instance like I have just done.
The book starts off with Ajax spearing the best Thracian warrior Acamas through the head, then Diomedes kills two guys, and then Euryalus slays another four dudes before we come to the line which you quote, in which three other Trojans meet their end.
So no, Pidytes is not a last name: it is simply the name of a warrior from Percote who is killed by "Ulysses." This is the only time Pidytes is ever mentioned as a character, as with many of his fellow dead. According to William Cowper's 1791 translation of your quote:
Dauntless Polypœtes slew Astyalus.
Ulysses with his spear
Transfixed Pydites, a Percosian Chief,
And Teucer Aretaön...
(Either by error or from preference, Cowper renders Pidytes as "Pydites.") For the characters' names and for place-names, Andrew Lang, Walter Leaf & Ernest Myers, in their 1893 version, use transliterations which are closer to the Greek original:
And stubborn Polypoites slew Astyalos, and Odysseus with spear of
bronze laid low Pidytes of Perkote, and so did Teukros to goodly
Going by Augustus Taber Murray's 1924 rendition:
And Polypoetes staunch in fight slew Astyalus, and Odysseus with his
spear of bronze laid low Pidytes of Percote, and Teucer goodly
On A.S. Kline's Poetry in Translation website:
Now rugged Polypoetes killed Astyalus, while Odysseus slew Pidytes of
Percote with his bronze spear, and Teucer noble Aretaon.
Are you saying that your edition/copy of Samuel Butler's Iliad translation uses Greek names for the gods but has "Ulysses" instead of "Odysseus"? Every version/edition of Butler's Iliad that I have seen uses Roman names for all the characters, whether divine or not.
Zeus is Jove throughout, except for an introductory note at the beginning of Book 14, where he is "Jupiter" (and a bewildering instance toward the end of Book 16 in which "Zeus" is mentioned, and which is surely a mistake[?]). Athene/Athena is always "Minerva," Ares is "Mars" and Aphrodite is "Venus." Similarly, in place of Odysseus we have "Ulysses."
Butler, who translated in 1898, like Cowper over a hundred years previously, was following a convention in certain circles in which, it seems, it was hip to be as Latinate/Roman as possible when dealing with the classics.* For the sake of consistency, then, while using the Roman names of the gods, Odysseus would uniformly be referred to by his Roman name: thus "Ulysses."
(It is on account of such Roman "hipness" that in present-day English-speaking popular culture, Hercules [mentioned in Butler's Iliad] is the better-known name for Herakles/Heracles, even while using Greek names for every other character [see, e.g., the 1997 Disney movie Hercules].)
*But maybe your Barnes&Noble Collectible Edition (of Butler) does this differently...?