I was discussing this with my wife yesterday and she asked if other half was always a human. I'm new to D&D (came into it with 5e 2014 sometime around 2021/2022) so I don't know. Were there rules for half-dwarf/half-orcs? (and so on) I thought it was always humans and she was incredulous about that.
As a PC race it has been half-human from 1e to 5e.
In the 1e MM it talks about half-orcs of other species specifically:
Half-Orcs: As orcs will breed with anything, there are any number of unsavory mongrels with orcish blood, particularly orc-goblins, orc-hobgoblins, and orc-humans. Orcs cannot cross-breed with elves. Half-orcs tend to favor the orcish strain heavily, so such sorts are basically orcs although they can sometimes (10%) pass themselves off as true creatures of their other stock (goblins, hobgoblins, humans, etc.).
In 5e 14 MM under orcs they have an entry too:
Ore Crossbreeds. Luthic, the orc goddess of fertility and wife of Gruumsh, demands that orcs procreate often and indiscriminately so that orc hordes swell generation after generation. The orcs' drive to reproduce runs stronger than any other humanoid race, and they readily crossbreed with other races. When an orc procreates with a non-orc humanoid of similar size and stature (such as a human or a dwarf), the resulting child is either an orc or a half-orc. When an orc produces young with an ogre, the child is a half-ogre of intimidating strength and brutish features called an ogrillon.
Ogrillons were originally orc-ogre cross breeds with their own stats in the 1e Fiend Folio. In 5e 14MM they are half ogres under the ogre entry with the other half being human, hobgoblin, bugbear, or orc.
