The Caspians or Caspi were ancient people dwelling along the southwestern shore of the Caspian Sea.The Caspians have generally been regarded as a pre-Indo-European, that is, a pre-Iranian, people and have even been identified by some scholars with the Kassites.Casspians inhabited in province of Guilan,northern Iran,where the language people speak is Guilaki that is greatly close to Mazandarani and and not that close,but still simular to Persian.The Caspians must therefore be considered either an Iranian people or strongly under Iranian cultural influence.
They were one of the branches of Kass people that were generally divided into two branches,namley Cassies and Casspies.In old Guilaki,Kass'es language, 'si' means mountain and 'pi ' means river or a river's delta.Casies were those who lived in mountains of Guilan and Caspies were people who lived beside the Casspian sea at the Sepidrud's delta.Sepidrud is a very great river that flows into the beautiful land of Guilan and divides it into two general parts,namely 'piepas (or biepas)' and 'piepish (or biepish)'.Biepas refers to the western regions of Guilan and means 'before the river' ,and biepish refers to the eastern regions of Guilan and means 'front of the river'.In these two terms,biepas/piepas and biepish/piepish, the word 'pie' has the same meaning as it has in the word Caspian, which is river.
And finally about the little suffix 'ian' as in Casspian,there are two general theorems that both seem two be correct and logical:
The first one says its the famous siffix in english that means ''someone or something of, from, or connected with a particular thing, place, or person '' as used in the following words for illustrartion: India and Indian,Iran and Iranian,America and American,etc.
The second theorem claims that that 'ian' at the end of the word is the wellknown suffix both in Persian and Guilaki that when added to the end of a word,makes it plural,just the same as 's' in english.So the word Casspian will be the plural form of Caspie.
This idea seems more acceptable when one considers that in Guilaki when name of a nation or city,etc, is made plural,it can refer to the land that the nation exists in or the region arround the city.And then the word Caspian will bear the meaning of "attributed to the land of Caspies".Of course there are other names in Guilan to support and illustrate this consept,like the word Guilan itself(guil+an=guilan) which means land of Guils and Guils are people who currently live in Guilan and are believed to be one of the sons of Kasses by many historians.Other examples are Deylaman(where Deylam people live,Saravan,Ghazian,Rashtian,Lahijan,etc that all are names of different placs in Guilan.

So Caspian sea is a Guilaki word and comes after the name of ancient Caspians

BAHRAM KARIMI