I never could understand why words like "chevalier" and "caballero" were derived from the Latin for "horse" which is "equus." Finally, in one of the books I read on the development of English (one of my hobbies), I found out about Vulgar vs Classical Latin. While not from that book, here's something from the web on the two latins.
Vulgar Latin - The Father of the Modern Romance Languages
No, Vulgar Latin isn't Latin filled with profanities or simply a slangy version of Classical Latin, although there certainly were vulgar words in Vulgar Latin.
Rather, Vulgar Latin is the father of the Romance languages: the Latin taught at schools, Classical Latin, is their grandfather.
Vulgar Latin, was spoken differently in different countries, where, over time, it became such familiar modern languages as Spanish, Italian, French, Catalan, Romanian, and Portuguese. There are others less commonly spoken.
The Spread of Latin
When the Roman Empire expanded, the language and customs of the Romans spread to peoples who already had their own languages and cultures. The growing Empire required soldiers be positioned at all the outposts. These soldiers came from all over the Empire and spoke Latin diluted by their native tongues.
The Latin Spoken in Rome
In Rome itself, the common people did not speak the stilted Latin that we know of as Classical Latin, the literary language of the first century B.C.
Not even the aristocrats, like Cicero, actually spoke the literary language, although they wrote it.
Evidence? We can say this because in some of Cicero's personal correspondence, his Latin was less than the polished form we think of as typically Ciceronian.
Classical Latin was, therefore, not the lingua franca of the Roman Empire, even if Latin, in one form or another was.
Difference Between Vulgar Latin and Classical Latin
Throughout the Empire, Latin was spoken in many forms, but it was basically the version of Latin called Vulgar Latin, the fast-changing Latin of the common people (the word vulgar comes from the Latin word for the common people, like the Greek hoi polloi 'the many'). Vulgar Latin was a simpler form of literary Latin.
•It dropped terminal letters and syllables (or they metathesized).
•It decreased the use of inflections, since prepositions (ad (> à) and de) came to serve in place of case endings on nouns.
•Colorful or slang (what we think of as 'vulgar') terms replaced traditional ones -- testa meaning 'jar' replaced caput for 'head'.
You may see some of what had happened to Latin by the 3rd or 4th century A.D. when a list of 227 fascinating "corrections" [basically, Vulgar Latin, wrong; Classical Latin, right] was compiled by Probus.
