Most of substantives and adjectives add “-es” instead “-s” when they end in stressed vowel, but not when they end in “e” or “o”. “-es” plurals have more prestige than “-s” one.
This one is used in colloquial language. For example:
–Maniquí/maniquíes, maniquís (mannequin)
–Bambú/bambúes, bambús (bamboo)
However, words as “papá” and “mamá” are exceptions because they add an “-s”.
Also, “maravedí” can make its plural with “-es” (maravedíes) or with “-s” (maravedís).
In addition, we use original plural form to foreign words. For example: sofás, bajás, bisturís, chacolís, gachís, ambigús, canesús, champús or menús.
NdR: This post has been written according to the rules of the Royal Spanish Academy or Real Academia Española (RAE)