in normal kro exists two clients ragexe and ragexeRE.
in zero client named ragexe too. but it related only for ragnarok zero server.
with hercules you can use one of this clients. Grf need use for same server from where you get client. Simplest way it install kro and use oficial updater or rsu to update it to latest version. Then download unpacked exe and patch it.
If you want one of old versions, here is harder. need find full client version bit older than you selected and apply all missing updated.
You may have different errors with lua or other files. Try search forums about this errors or ask in irc or discord.