I suppose it kinda depends on 2 things in the OP's questions.
1. Do guilds forced to disband and remake due to the merger's naming being ugly for guilds that have common names on other servers still count as "oldest" ones or is the OP thinking they don't count
2. What is the definition of "existing" TGB from Teos is one of the extremely old guilds of the server (aka a day1 one) and it still exists, but solely for the reason of keeping the name alive.
Personally I think those forced to remake should still count, and for a guild to be existing it needs "active" players not just a guild name on the guild list
-This is true for a lot of active guilds. -Eternity- unwantingly became -Eternity--2 after Etain, Gaia, and teos Merged, and then after Lailah also merged, it became -Eternity---2 (wth lol), so a lot of us (original guilds) HAD to disband in order to reclaim the original guild name.
-Active guilds I guessing is what the Op is looking for, not just an old guild name without active members, but dont quote me on that.