I'm having the same problem. xD That was so darn long ago - I'm really not sure whether I'd have used something Yoshi-related.. or my then-usual Tashahi. Or even something else entirely. And if it was Tashahi, I have no idea what my pa-ssword was.
Probably something ridiculously easy though. o3o
Hm. Or maybe I didn't even register at all. I'm pretty sure it was possible to adopt a Pokeball and only provide your email.. Then they'd send the code and voila.
/ramble