The word Oni actually covers a larger expanse of creatures then just the western idea of Goblins. It covers what would be considered Orges and trolls and a few things not known to the wesr. It's evolved to a general meaning of monster like Kaiju. The typical Oni or goblin had bright hair like Red long noses and a horn in it's head. The fact the first westerns they met also had long noses and some with blonde or red hair didn't quite help.
The Kappa or water spirite was a dark brown skinned being with webbed feet and hands. In their heads was a depression that it keeps water in. They were mostly non violent yet mischeivious tricksters who greatest joy was to steal your innard through your anus. To ward them off if you met one you'd bow. When the kappa bowed the water in it's bowl head would spill out and it would then rush back to refill it. Truthfully most of legend is correct so only the foolish ever are harmed by them. If you remeber your Japanese manners you don't have a thing to worry about.
The Tengu were a type of Oni to live in forests and mountains. They had long noses and dislike of all humans. So as humans made their way into thier areas they ran away rather then deal with them and currently few are known to exist let alone show themselves. Their numbers are strongest along the still semi undeveloped northside of the Main island, Honshu.