They aren't part of historical and present day Nigeria and the Yoruba, the largest and dominant ethnic group, wouldn't tolerate their former Fulani slaves squatting on their territory.
edit: well I guess they do allow them to hang out in the north where the other African sandniggers hang out but I've never heard a Nigerian say anything nice about them.
Again, they were in the region now known as Nigeria before the region was colonized, and they most certainly are part of present day Nigeria. This kind of fails to matter because Nigeria's borders weren't exactly discussed by the the tribes caught up in them, which was the case for all the other African countries. If there's anything that you can say about the Fulani, it's that they're a nomadic tribe spread out across West/North/Central Africa, but even then, some of them settled into what is now northern Nigeria before the region was colonized by the British.
I don't really know anything about the Yorubas once having Fulani slaves-- that's probable, I suppose, though I find it odd that, of the slaves bought from Africa and brought into America,
none seem to have any Fulani, or Hausa, or Igbo heritage.
Also, it's the
Hausa that are the
plurality sub-ethnic group in Nigeria. That's followed by the Yoruba, then the Igbo (both in roughly equal measure), then the Fulani-- but the Hausa and Fulani have miscegenated enough that many sources will lump them together, and people in general will generally refer to them as just "Hausa" without distinction (or rather, I should say that I've heard more mention of the Hausa-- as rare as that may be-- versus that of the Fulani, of which I've heard none).
You're hardly ever going to find a Hausa, talk less of a Fulani, in America-- Nigerian immigrants in America are principally Yoruba and Igbo, both of which are both majority Christian (the Igbo more so) as well as substantially westernized versus the Hausa-Fulani. You'll also hardly hear Nigerian immigrants in America (of any length of time) talk about the Hausa or Fulani
at all, unless they're talking about politics, and in particular, about the current president-- but I never heard anybody deride that guy for being Fulani (well, Hausa-Fulani-- his mother had Hausa heritage, his father was a Fulani chieftain).
I could imagine that the
Igbo may have some animus against the Hausa, because the Hausa (and Yoruba) wouldn't let them properly secede back in the 60s, and capped off the Nigerian civil war by starving Igboland into submission. That, and since then-- to my knowledge-- you won't find an Igbo president or vice president, almost certainly because of Hausa and Yoruba remembrance of that war. But my impression is that said animus is both confined to the older generations (as in, those who witnessed the horror of the war), rare, and even dying out before they do.