Octopi, Platypi, and Hippopotami, Oh My!

I'm a web content editor by day, but at night I turn into one wild and crazy gal. Well, if you count random thoughts about grammar rules while in the shower as "wild and crazy."

So here's what I was thinking last night:

If the plural for nucleus is nuclei;
And the plural for syllabus is syllabi;
And the plural for radius is radii;
the plural for octopus is octopi;
And the plural for hippocampus (seahorse) is hippocampi;
And the plural for platypus is platypi;
And the plural for hippopotamus is hippopotami;

Why isn't the plural for walrus walri?

All of these, except hippocampus, also has an -es plural option, but why doesn't walrus have an -i plural? For the record, the second plural option for walrus is—wait for it—walrus. I did not know that until I looked it up.