Solved "A mysterious cry echoed up from the depths"

[Kyustorm]

What is this?? My game keeps saying this, and then "I wonder what it could be?" right after. It happens every time I enter Union Cave, and I just started this Online file..it's sort of creeping me out.. I don't know what Pokemon / what it is, could someone please tell me? <>

 

[Enethil]

On Fridays there is a peculiar pokemon in union cave. You can only reach it if you got surf. As for what the pokemon is, it will be a surprise ^^

 

[Kyustorm]

Oh no!! I've gotta hurry to get surf then today, maybe... D:
Haha, alright, thanks, that calmed me down a bit, it was really freaking me out..

 

[Tornado9797]

Oh wow that was close! I almost missed it, thanks for the reminder.

 

[panguinix]

Just to make sure is the pokemon rideable on the water and known for it?

 

[dylanwicorek]

Yes

 

[panguinix]

Ok i dont need it then

 

[Kyustorm]

I just realized what it was, and I had NO clue that you could catch it (Always thought you had to receive it like you did in other generations) even though I played Gold and Soul Silver <<
Commence the biggest facepalm ever..

 

[SACooper95]

Ooh... thanks for the hint! That pokemon's on my top 10 list!