It is mentioned in the Zohar 276a
וְרוֹכֵב עַל חֲמוֹר. עָנִי אִיהוּ תַּמָּן בְּסִימָן, עֵרוּבִין נִדָּה יְבָמוֹת, וּשְׁאַר מַתְנִיתִין בִּכְלָל. וְלָא אִתְקְרֵי תַּמָּן מֶלֶךְ, עַד דְּרָכִיב בְּסוּסְיָא דִּילֵיהּ כְּנֶסֶת יִשְׂרָאֵל.
And riding upon an donkey" (Zechariah 9:9). Ani stands for the Mishnah orders of Eruvin, Nidah and Yevamot, [Ani also means "poor"] and the rest of the Mishnah in total. [ As long as a person does not know the mysteries of the Torah, but only knows the revealed Torah, one is "poor" in knowledge and it is like he is "riding on a donkey", which is the aspect of the Tree of Knowledge of Good and Evil. However, Hashem] is not considered a King until He rides on His horse, which is the Congregation of Yisrael (Sefaria translation and notation)
I also saw that Rav Yaakov Emden in his Mitpachas Sefarim 4 also notes this abbreviation.