Ah i think the real reason i hate DBZ is because how the english dubbers literally ruined it for me stupid dubbers!
then watch it in japanese...

But, if kyubi were always around thru out history the whole world would be destoried I figure kyubi needs to devour a human soul to take form.
kyubi has been alive for a long time...but he is most probably like a summoned creature...eg gamabunta, manda, etc...he is in another place...and needs to be summoned to this world to do any damage..which is what might have happened. but this is still needed to be explained.