Translation of the Fan Theory from 2ch Regarding the Ending of Terranigma

Carried by a bird, Ark flew above the whole world, from one corner to the other, finally descending to Storkolm.

There, Overworld Ark greeted him: “Yo, finally you came!”
Underworld Ark was dumbfounded: “Huh? Is this part of the dream?!”
Overworld Ark: “You’re not dreaming. Once again, you came into this world.”
Underworld Ark: “But wasn’t my duty finished? Wasn’t my form supposed to return to the void?”
Overworld Ark: “Have you already forgotten? You also have Light Gaia’s blood flowing in you. Now then. Let us fuse one last time. You still have some unfinished business, don’t you?”
Underworld Ark asked “What is this?” as a dazzling light emanated from Overworld Ark’s body. The next moment, Underworld Ark said: “Huh? My original form returned?!”
Overworld Ark’s voice: “My duty is now finished. From now on you will have to walk your own path. Now, go!”
Underworld Ark: “… Thank you…”

After that…

Overworld Elle: “Who could it be at this hour? It’s almost midnight…”

~ THE END ~

What's the bird carrying Ark around?

As you can see, in English it was translated as a gull, which could mean any number of shore birds (source). If you think that's a bit vague, look at the Japanese version:

Here it is simply called wataridori, or migratory bird. Even the English translation hints to this in dialogues, showing how gull was just a short word to stand in for migratory bird which was impossible to use, given the cartridge limitations for the recurring names.

But there's more! On the one and only Japanese board 2ch someone pointed out that you'd need a really big migratory bird for this sort of task. Well, good thing there's already such a bird that is portrayed in both Western and Eastern mythology as carrying humans or transferring souls. Its name? Shirasagi, a term denoting all big migratory birds with long beaks and white plumage, such as the stork or the white heron.

And now you know what's that bird which carries Ark around in a bag, just like a baby (subject for infinite jokes among Japanese fans).

Even more, you can see how Ark's metamorphosis in the end all ties in neatly with this. After all, storks are symbols of the soul and its next voyage. The ancient shirasagi ceremony wonderfully portrays this role, with the dancers momentarily transformed themselves in the majestic white bird.

Let's just hope Ark turned back into a human after he reached Elle's house in Storkolm (lit., stork islet). It would be a tad hard to enter the door with such a long neck.

2ch short fan fiction for the ending

A user on the Japanese 2channel posted this version of the ending. I like it. It makes sense considering what we’re told in the game and it’s a happy ending.

～エンディング～
鳥になったアークは一通り世界を飛び回って最後にストークホルムにやって来た。
そこには、表のアークの幻影があった。表アーク「やあ、来たね」
裏アーク「？？？これは夢・・？？」
表アーク「夢じゃないよ。君は再びこの世界にやってきたのさ」
裏アーク「でも俺の役目はもう終わって、この体は消えるはずだろ？」
表アーク「忘れたのかい？君にはライトガイアの血も流れているんだよ。
さあもう一度融合しよう。君にはまだやらなければいけないことが残ってるだろ？」
裏アーク「わっ！！」まばゆい光を放つ表アーク。そして次の瞬間裏アーク「・・・・・！！！これは？？体が元に戻った！？」
表アークの声「僕の役目はもう終わった。これからは君自身の人生を歩んでいくんだ。さあ！」
裏アーク「・・・・ありがとう・・」そして・・・
表エル「誰かしらこんな夜中に。もう夜中の12時近いのに・・・どなたですか？」ＴＨＥ　ＥＮＤ