Yarii is the plural form of Yaris.

[earljail]

My wife and I say Yarii for plural, Yaris for singular.

We also say Matrices when we're talking about more than one Toyota Matrix.

Originally Posted by kimona

Mr. and Mrs. Jarvis live next door. Together, they are the Jarvises.

Like Jarvis, Yaris is a name, so you have two Yarises.

Sure, but English has irregular plural forms.

Sing - Plural
criterion - criteria
sheep - sheep
brother - brethern (old)
tooth - teeth
forum - fora

Besides, Yaris is a made-up word, which means nobody can be wrong. Wikipedia suggest that it was from from "ja"(German for yes) and "Charis"(a Greek god)