They are probably people that Harry didn't recognize or just didn't know well enough to comment. I'm sure he would have said something if it was any Gryffindor 7th year boy or if it was Lavender or Parvati. It could be one of the unidentified Gryffindor girls, or memebers of other houses that Harry didn't really know. It also didn't say they were human, so it's possible that a house elf or two were killed. Harry didn't know all the order members, so its quite possible it was some of them.
It is likely most of them were adults of at least seventeen years old, as Harry Potter did not see anyone he recognised dead, besides those he named. It is likely that half of the fallen would have been fellow students, the rest possibly members of the Order of the Phoenix, staff members, or residents of Hogsmeade. There may well have been even more casualties than just fifty, but the bodies of these may not have been collected at that point, as Colin Creevey's body was discovered out on the school grounds by Oliver Wood and Neville Longbottom and as Hogwarts is such a large place it is unlikely that all bodies were retrieved and brought back to the Hall during that phase of the battle.