“Ah, Poko-tan has logged in!” (あ、ぽこたんインしたお！, A, Poko-tan in shitao!)¹⁾ was a Final Fantasy XI-related kopipe that was originally posted in a linkshell²⁾ complaint thread on March 7, 2004.³⁾ This later sparks the whole '-o' (～お) trend on /news4vip/, as seen with Naito Horizon.

In the original post, the user had decided to talk about how they felt 'grateful' about coming into contact with a nice and laid-back⁴⁾ linkshell that was fairly populated. However, the chatlog provided shows the user being interrupted by a 'Tarutaru' who points out that “Poko-tan has logged in!” to the linkshell, and everyone proceeds to welcome 'Poko-tan' who might be implied to be playing as an 'Elvaan' character.

93 ：既にその名前は使われています：04/03/07 03:11 ID:1FBId3zq
うちのｌｓは、平均20～25ほどで最大84人いるぽ
それでいて、マターリした雰囲気のいいｌｓ
今でもＦＦ続けてるのは、このｌｓに入ったおかげだなと思う。
おかげで毎日仕事から帰って、インするのが楽しみでしょうがないｗ

おいら「おいすー＾＾
たるＡ「あ、ぽこたんインしたお！」
一同「こんばんわ～＾＾」
エルＡ「よーし今日もどっかいくかー＾＾」
一同　「おー＾＾」

93 ：That Name is Already in Use：04/03/07 03:11 ID:1FBId3zq
Our linkshell averages between 20–25 people, with a maximum of 84.
Despite this, it's such a nice and laid-back linkshell.
I think the reason I still play Final Fantasy is because I was blessed with this linkshell.
Thanks to them, I can't wait to log in after coming home from work every day, lol.

Me 「What's up? ＾＾
Tarutaru A 「Ah, Poko-tan has logged in!」
Everyone 「Good evening~ ＾＾」
Elvaan A 「Alright, where are we going today? ＾＾」
Everyone 「Oh ＾＾」

The post appears to have been an off-topic 'humblebrag' of sorts, which later became a kopipe as users kept parodying and spamming it. At some point, the aforementioned 'Tarutaru' was rendered in text artwork as a character emerging from the ground with poop on its head, then people were confused on who 'Poko-tan' was, thus the 'Poko-tan announcing themselves' interpretation.

“Moko-tan has logged in!” (もこたんインしたお！, Moko-tan in shitao!) is the Touhou Project variation of the meme. However, the context was changed to Mokou emerging from the hatch, Keine acknowledging Mokou, and everybody greets her as the story typically goes. Since then, the 'Moko-tan' (もこたん) nickname has remained a relatively popular nickname for Fujiwara no Mokou in the Japanese community.

　　　　　 ,-へ,　 , ﾍ
　　　　／,ヽ_,_i=/__,」
　　　 /　,'　　 ｀ｰ ヽ ﾊﾟｶ
　　　/ ∩〈」iﾉﾊル.!〉　　　Ah, Moko-tan has logged in!
　　 /　.|i L>ﾟ ∀ﾟﾉiゝ＿
　　/／i>i ir^i ｀T´i'i| ／
　　"￣￣￣￣￣∪

• Square Enix would later include a reference to the line in the Japan-exclusive Hanjuku Hero 4.
• In the .hack series, there's a chaotic player killer in .hack//G.U. named 'Magical Pokotan' (拷鉄魔法少女ぽこたん, Goutetsu Mahou Shoujo Poko-tan) who speaks with a '-dao' (～だお). Her offline name is 'Matsumi Hori' (堀 真澄, Hori Matsumi). She is voiced by Yōko Honna.

• Poko-tan (Taru A) on the 2ch AA Illustration Project
• Ah, Poko-tan has logged in! on Niconico Encyclopedia
• Moko-tan has logged in! on Pixiv Encyclopedia
• Moko-tan has logged in! on Niconico Encyclopedia