What is the tahash תחש ?

וְנָ֨שְׂא֜וּ אֶת־יְרִיעֹ֤ת הַמִּשְׁכָּן֙ וְאֶת־אֹ֣הֶל מוֹעֵ֔ד מִכְסֵ֕הוּ וּמִכְסֵ֛ה הַתַּ֥חַשׁ אֲשֶׁר־עָלָ֖יו מִלְמָ֑עְלָה וְאֶ֨ת־מָסַ֔ךְ פֶּ֖תַח אֹ֥הֶל מוֹעֵֽד׃

Numbers 4:25

They shall carry the tapestries of the Mishkon and [of] the Tent of Meeting, its covering and the covering of tachash which is on top of it, and the screen for the entrance to the Tent of Meeting,

The skin of the Tachash (תחש in Hebrew) was used in many places in the tabernacle (mishkan / משכן in Hebrew). Here we see that it was part of the tapestries of the Tabernacle and of the tent of meetings.
What is this animal that the Torah calls tachash?
Some translations translate tachash as dolphin but that's not what I'm able to find in commentaries.
Here is another mention of tachash earlier in the Torah when talking about all the things that the children of Israel should donate for the building of the mishkan.

שמות כ״ה:ה׳

וְעֹרֹ֨ת אֵילִ֧ם מְאׇדָּמִ֛ים וְעֹרֹ֥ת תְּחָשִׁ֖ים וַעֲצֵ֥י שִׁטִּֽים׃

Exodus 25:5

Red[-dyed] ram’s skins, tachash skins, and acacia wood.

תרגום אונקלוס על שמות כ״ה:ה׳

וּמַשְׁכֵי דְדִכְרֵי מְסַמְּקֵי וּמַשְׁכֵי סַסְגּוֹנָא וְאָעֵי שִׁטִּין:

Onkelos Exodus 25:5

Red[-dyed] ram’s skins, sassgavna skins, and acacia wood.

Onkelos translates tahash into Aramaic as sassgavna. We're not familiar with that word either but Rashi brings an explanation.

רש"י על שמות כ״ה:ה׳:ב׳

תחשים. מִין חַיָּה, וְלֹא הָיְתָה אֶלָּא לְשָׁעָה, וְהַרְבֵּה גְּוָנִים הָיוּ לָהּ, לְכָךְ מְתֻרְגָּם סַסְגּוֹנָא שֶׁשָּׂשׂ וּמִתְפָּאֵר בִּגְוָנִין שֶׁלּוֹ (שבת כ"ח):

Rashi on Exodus 25:5:2

תחשים TACHASH was a kind of wild beast. It existed only at that time (when Israel built the Tabernacle). It was multi-coloured and therefore it is translated in the Targum by ססגונה, and it is so translated because it delights (שָׂשׂ sass) and prides itself in its colours (גונא gavna) (Shabbat 28b; Midrash Tanchuma, Terumah 6).

גִוּוּן in modern Hebrew means hue.
So Rashi says that the tahash was an animal that delights people thanks to its multi-colored skin! If you look at the sources that rash brings there's more to it!

שבת כ״ח ב:ו׳

מַאי הָוֵי עֲלֵהּ דְּתַחַשׁ שֶׁהָיָה בִּימֵי מֹשֶׁה? אָמַר רַבִּי אִלָּעָא אָמַר רַבִּי שִׁמְעוֹן בֶּן לָקִישׁ, אוֹמֵר הָיָה רַבִּי מֵאִיר: תַּחַשׁ שֶׁהָיָה בִּימֵי מֹשֶׁה בְּרִיָּה בִּפְנֵי עַצְמָהּ הָיָה, וְלֹא הִכְרִיעוּ בּוֹ חֲכָמִים אִם מִין חַיָּה הוּא, אִם מִין בְּהֵמָה הוּא. וְקֶרֶן אַחַת הָיְתָה לוֹ בְּמִצְחוֹ, וּלְפִי שָׁעָה נִזְדַּמֵּן לוֹ לְמֹשֶׁה וְעָשָׂה מִמֶּנּוּ מִשְׁכָּן וְנִגְנַז.

Shabbat 28b:6

The Gemara asks: What is the halakhic conclusion reached about this matter of the taḥash that existed in the days of Moses? Rabbi Ela said that Rabbi Shimon ben Lakish said that Rabbi Meir used to say: The taḥash that existed in the days of Moses was a creature unto itself, and the Sages did not determine whether it was a type of undomesticated animal or a type of domesticated animal. And it had a single horn on its forehead, and this taḥash happened to come to Moses for the moment while the Tabernacle was being built, and he made the covering for the Tabernacle from it. And from then on, the taḥash was suppressed and is no longer found.

מדרש תנחומא, תרומה ו׳:א׳

‎וְזֹאת הַתְּרוּמָה וְגוֹ'. וְעֹרֹת אֵילִם מְאָדָּמִים וְעֹרֹת תְּחָשִׁים. רַבִּי יְהוּדָה וְרַבִּי נֶחְמְיָה. רַבִּי יְהוּדָה אוֹמֵר: חַיָּה טְהוֹרָה גְּדוֹלָה הָיְתָה בַּמִּדְבָּר וְקֶרֶן אַחַת הָיָה לָהּ בְּמִצְחָהּ, וּבְעוֹרָהּ שִׁשָּׁה גְּוָנִים, וְנָטְלוּ אוֹתָהּ וְעָשׂוּ מִמֶּנָּה יְרִיעוֹת. וְרַבִּי נְחֶמְיָה אוֹמֵר: מַעֲשֵׂה נִסִּים הָיְתָה, וּלְשָׁעָה שֶׁנִּבְרֵאת, בָּהּ בַּשָּׁנָה נִגְנְזָה. וְעֹרֹת תְּחָשִׁים, לָמָּה? דִּכְתִיב: אֹרֶךְ הַיְרִיעָה הָאַחַת שְׁלֹשִים בָּאַמָּה. מִי מֵבִיא לְךָ יְרִיעָה שֶׁל שְׁלֹשִׁים אַמָּה. אֶלָּא מַעֲשֵׂה נֵס, לְשָׁעָה שֶׁנִּבְרֵאת נִגְנְזָה.

Midrash Tanchuma, Terumah 6:1

This is the offering … and rams’ skins dyed red, and sealskins (Exod. 25:3). R. Judah and R. Nehemiah discussed this verse. R. Judah said: It was a large pure animal, with a single horn in its forehead and a skin of six different colors that roamed the desert. They captured one of them and from its skin made a covering for the ark. R. Nehemiah contended that it was a miraculous creature He created for that precise moment, and that it disappeared immediately thereafter from earth. Why is it called orot tahashim (“sealskins,” lit. “skins of tahashim”)? Because the verse states: The length of each curtain shall be thirty cubits (Exod. 26:8). What known animal could supply enough skin for a curtain of thirty cubits? It must, indeed, have been a miraculous creation, which disappeared (immediately after it was created).

So the tahash was an animal with a single horn and a multi-colored skin!
Some say that it was a miraculous creature that was created precisely for that moment, when it's used in the mishkan.
Sounds a lot like a rainbow unicorn to me!