The plural of Prius is Priuses, because names take regular plurals, as in the Childs, the Manns and the Maple Leafs.

The claim has two parts: (a) the correct plural of the car name 'Prius' is 'Priuses', and (b) the reason is that names take regular plurals, shown by 'the Childs', 'the Manns' and 'the (Toronto) Maple Leafs'. Judge it as of 2019-12-21 by standard English usage: what dictionaries, usage guides and linguistic accounts of plurals say, and what people commonly write. A manufacturer's promotional or 'official' plural does not by itself make a plural correct in English. TRUE: usage authorities or corpus evidence treat 'Priuses' as the standard or dominant plural, AND linguistic sources support the rule that proper names (including names built from irregular nouns such as child, man or leaf) take regular -s/-es plurals, with the examples given matching that rule (e.g. the Child family is 'the Childs', the hockey team is the 'Maple Leafs'). MOSTLY TRUE: both points hold but one example is slightly off (e.g. it is disputed or not quite an instance of the rule). MIXED: one part holds and the other does not, e.g. 'Priuses' is standard but the stated reason is not the accepted explanation, or the reason is right but another plural is at least as standard. MOSTLY FALSE: usage authorities treat another plural (e.g. 'Prii') as the standard form, but 'Priuses' is still widely used or accepted. FALSE: authorities reject 'Priuses' as the plural and the claimed rule about names is shown to be wrong.

Set before the research began.

1. E1 The Grammarphobia Blog: Three little Wendys Grammarphobia · 29 October 2013

   “In general, names are pluralized the same way as ordinary nouns—except for irregular nouns like “sheep,” “fish,” “children,” and others.”

2. E2 The Grammarphobia Blog: Three little Wendys Grammarphobia · 29 October 2013

   “A family named “Child,” for example, would be pluralized as “the Childs.””

3. E3 11 Brand Names With Confusing Plurals Mental Floss · 22 January 2014

   “But plain old English Priuses seems to be the more common choice in real life.”

4. E4 Toyota Prius - Wikipedia Wikipedia

   “Despite the "official" plural form used by Toyota USA, "Priuses" is widely used in English.”

On the plural, E3 says "Priuses" is the more common choice in real life, and E4 says "Priuses" is widely used in English despite Toyota's "official" form. That fits the criterion that usage, not the manufacturer, decides the plural. On the reason, E1 says names take the same plurals as ordinary nouns but not the irregular ones like "children", and E2 gives "the Childs" as the plural for a family named Child. Both match the claimed rule and example. The evidence does not discuss "the Manns" or the "Maple Leafs" by name, but they fall under the stated rule and nothing in the evidence contradicts them.