As with several other sections, this one will probably be short. Why? Well, new, Japanese-themed monsters was another thing that Oriental Adventures did right. Most of the new beasties that they introduced were based on myths and legends from ancient Japan, and many had Japanese names even if they weren't authentic legends.
Because of this, it is highly recommended that you get Oriental Adventures if you want to throw crazy legendary creatures at them like Oni and Hebi-no-Onna. And some of the Shadowlands Oni, while created for a Rokugan campaign, are all unique, creepy monsters that could easily crawl out of Jigoku.
However, you do not need Oriental Adventures, and even if you have it, you don't have to constantly throw Eastern creatures at them. Because of the way the history of the Empire works, it's almost like the future of any other DnD society. So while you might encounter a raging, three-eyed oni, it's plausible and possible for the PCs to run into a more classic DnD creature (like a Displacer Beast). In fact, it's possible to run a campaign without Oriental Adventures and any of its monsters, but I don't recommend it.
New Names for Old Monsters
Japanese folklore already has a lot of monsters, including several hundred that were illustrated. While many of these monsters are unique to Japan, many of them have equivalents in the Monster Manuals. Below is a list of traditional Japanese monsters that were created already, with their Monster Manual names in parenthesis, as well as some variant rules to make them more authentic.
Links to any new, Eastern themed monsters that I create.