ויקראו שמו עשו (תולדות כה:כה)

Rashi says: “הכל קראו לו כך” – everyone named him like that; therefore, it’s in plural form.

Chazal says that Eisav means “הא שוא שבראתי בעולמי” – this is the falsehood I created in this world.  Eisav symbolizes שקר – falsehood, and since many people are attracted to falsehood, it’s in plural form.

Yakov represents אמת – truth, as it says “תתן אמת ליעקב” – I gave truth to Yakov, and by Yakov’s name it says “ויקרא שמו יעקב” – and he named him Yakov, in the singular form, since the truth has fewer followers and only individuals are attracted to the truth.