In English, some nouns have regular plural forms ending in -s or -es and fewer are irregular. Fish in the plural is still fish while child becomes children.
In Spanish, nearly all nouns are regular, adding -s or -es. The rule is even simpler than in English (-es is for words that end in consonants and accented vowels other than é, s for all others). Also, words that end in z change the z to c in the plural to retain a pleasing sound in all regional accents.
There are some words borrowed from other languages that have irregular plurals. In Mexico, the weekly markets are called tianguis (singular: tianguis plural: tianguis) after the Nahuatl word for the same weekly markets (often in the exact same places). Tianguis has been a common Spanish word since around A.D. 1520 but it still seems like a piece of traditional pre-hispanic Mexico.
Are there any native or fully absorbed Spanish nouns with irregular plurals?