Within a year, Britain split off the area east of the Jordan River, calling it Transjordan (and the part west of the Jordan River - now mostly Israel - was then called Cisjordan) and later setting up a Saudi family to rule it as the Kingdom of Jordan; Transjordan amounted to approx 78% of the area of the original Palestine Mandate. A year after that, Britain handed the Golan Heights over to the French Mandate for Syria.
So Jordan is most of what was Palestine, and apart from its "royal family" (made royal only by British political appointment), its population is ethnically indistinguishable from the Arabs west of the Jordan ... a fact that the Jordanian govt emphasized repeatedly over the years.
Of course, if you're old enough to remember the Middle East before 1967, you remember that, before Israel won the West Bank and Gaza in the Six Day War, nobody ever mentioned a "Palestinian" people. For the 19 years that Jordan held the West Bank and Egypt held the Gaza Strip, there was no mention of another Arab nationality with any entitlement to an inch of that land; Jordan claimed the West Bank for itself, imposed its own laws and taxes and troops on the Arabs there, and Egypt did much the same on the Gaza, and the UN (and the White House and the rest of the world) did not talk about "Jordanian-occupied West Bank" or anything like that, nor did it breathe a word suggesting that the Jordanians were squatting where another Arab nation had a right to be. Only after the Jordanians and Egyptians were ousted did this mythical "Palestinian nation" get mentioned ... as an excuse to pry the land away from Jews. Even then, the crucial Arabs were sluggish about it; Jordan persisted in claiming the West Bank for itself until about 1988.