Well it is explained below. Turns out that Finns had another word "Marsu" for walrus, which also have tusks. So therefore they started using a similar word for a (somewhat) similar animal.
As there are no walruses in Finland, it was nearly as mysterious a creature as an elephant to Finns, who mostly encountered only walrus tusks and walrus hides through trading.[2]
Russian borrowed the word for walrus морж from some Sámi or some Finnic language, as the Russian expansion to the Arctic Ocean is historically much later than the Sámi and Finnic settlement in the area. [2] From Russian, the word got borrowed into French as morse.
Btw, you had a typo in the name of the animal. marsu means "a guinea pig" in Finnish, borrowed from Swedish marsvin (literally "a sea pig"), which in turn comes from German.
[1](https://www.sgr.fi/sust/sust266/sust266_aikio.pdf) [2](https://journal.fi/virittaja/article/view/40611)
The question is, why those two languages and only those two?
What's worse, the Slavic word dates to proto-Slavic times, it's one of the basic animal words like "deer", "rabbit", "cow", etc.!
So the only languages in Europe except the other language in Europe? :)
> from Greek elephas (genitive elephantos) "elephant; ivory," probably from a non-Indo-European language, likely via Phoenician (compare Hamitic elu "elephant," source of the word for it in many Semitic languages, or possibly from Sanskrit ibhah "elephant").
So the Greek word is not necessarily native to Greek.