Why is he called "Yeser ha'Yishmaeli"?
Yevamos 77a: He girded his sword like a Yishmaeli, and threatened to stab anyone who will contest his tradition from Shmuel ha'Navi that an Amonis or Mo'avis is permitted.
'Rashi': In Shmuel II, 17:25 he is called Yisra ha'Yisraeli. Here it says Yishmaeli, for he lived in Eretz Yishmael. Also "Beis Oved Edom ha'Giti" (was called Giti based on his location), "v'Aviv Ish Tzuri" (from Tzur). Radak - [the verse] called him Yisraeli, lest one think that he was called Yishmaeli because he was from Yishmael's seed.
Malbim: Perhaps he was a Yishmaeli convert. After conversion, they changed his name to Yisra ha'Yisraeli. Yalkut Shmuel 127 says that after he converted, Yishai gave his daughter to him 1 .
Malbim: Below (21), 'Rashi' said that "Ba Chetzron" is without marriage. Likewise, we can say that "Asher Ba El Avigayil" was before he converted, without marriage; the child is Kosher.
