Why is walri not the correct pluralization of walrus?
Only some words of Latin extraction ending in -us, which were second declension nouns in Latin, take -i as plural.
Walrus comes from Dutch, and is akin to Danish and Norwegian hvalros. It is not a Latin second declension noun, so there is no reason it would be pluralized with -i.
The plural of walrus is walruses.
Edit: I just want add an additional note to clarify that even in cases where an English word does come from a Latin second declension noun, its plural might not necessarily end in -i. For example, the plural of campus is campuses not campi, and the plural of bonus is bonuses not boni.