This question may seem to be obvious, given Rift Herald's appearance, but Riot has never stated this directly. Also, the Rift Herald doesn't have a name with an apostrophe.
The Baron Nashor has been confirmed as a Void creature. http://leagueoflegends.wikia.com/wiki/Void#Trivia
Umbral Regent1 votesBaron Nashor's name doesn't have an apostrophe, either. So, yeah, it's more than likely that, given the Rift Herald's placement on the map and his features, he's a Voidspawn too.
YumaS2Astral1 votes[{quoted}](name=Umbral Regent,realm=NA,application-id=6kFXY1kR,discussion-id=rXAZbpol,comment-id=0000,timestamp=2015-11-20T13:46:51.051+0000)
Baron Nashor's name doesn't have an apostrophe, either. So, yeah, it's more than likely that, given the Rift Herald's placement on the map and his features, he's a Voidspawn too.
On the spectator mode, when a team slay the Baron Nashor, his name is displayed as "Baron Na'Shor" - this happens on the Portuguese language but I don't know if this can happen on other languages, too.
GreenLore1 votesThey actually did state it directly. It is confirmed that the rift herald is actually a scuttle crab that was corrupted by the void.
YumaS2Astral1 votesThey actually did state it directly. It is confirmed that the rift herald is actually a scuttle crab that was corrupted by the void.
source plz