œ = german ö = o+e, the ProtoEldarin form of an o assimilated to i (from the plural: horseman=rochben, in the early time the plural was made by adding an "i" and the vowels were affected to it -> o+i=oe, e+i=i); in the later evolution, œ became e, so rœchbin is actually the archaic form; "modern" Thirdagesindarin it would be different, normally it should I think be with e, but in this case I think it's just rochbin as only the "bin"="men" is plural, "horsemen", not "horsesmen" 😛