Why does Rashi say תנין is a snake, not a croc?

Question

Rashi on Shemot 7:10:

‏ "ויהי לתנין" - נחש

The pasuk:

וַיָּבֹ֨א מֹשֶׁ֤ה וְאַהֲרֹן֙ אֶל־פַּרְעֹ֔ה וַיַּ֣עַשׂוּ כֵ֔ן כַּאֲשֶׁ֖ר צִוָּ֣ה יְהוָ֑ה וַיַּשְׁלֵ֨ךְ אַהֲרֹ֜ן אֶת־מַטֵּ֗הוּ לִפְנֵ֥י פַרְעֹ֛ה וְלִפְנֵ֥י עֲבָדָ֖יו וַיְהִ֥י לְתַנִּֽין׃

Why does Rashi translate תנין as נחש, a snake? In modern Hebrew, a תנין is a crocodile.

I know that you can't really bring a proof from MIH on BH, but there is an existing word for "snake" (נחש).

Is it possible that Rashi simply didn't know about crocodiles? According to Wikipedia, Crocodiles apparently aren't too common in France:

Answer 1

Lubavitcher Rabbi Zatzal explains that Rashi says that תנין is a snake since further in Posuk 7:15 it says וְהַמַּטֶּה אֲשֶׁר נֶהְפַּךְ לְנָחָשׁ תִּקַּח בְּיָדֶךָ, obviously it turned into a snake. He leaves open the question as to why the Pasuk refers to it as a תנין if it is a snake.

Tiferes Yisrael says that the hidden letters of Nachash are תנין.

נ"ן חי"ת ש"ן