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כְּתִיב לְמַעַן יָנוּחַ שׁוֹרְךָ וַחֲמוֹרֶךָ וְגוֹ', הֲרֵי כִּי הִזְהִירָה הַתּוֹרָה, שֶׁגַּם הַבְּהֵמָה שֶׁל יִשְֹרָאֵל תָּנוּחַ. וְלֹא בְּהֵמָה בִלְבַד, אֶלָּא כָּל בַּעֲלֵי חַיִּים. וְלָכֵן אָסוּר לְהַנִּיחַ לִבְהֶמְתּוֹ שֶׁתּוֹצִיא אֵיזֶה מַשָּׂא, שֶׁאֲפִלּוּ יָצְאָה מֵאֵלֶיהָ לִרְשׁוּת הָרַבִּים וְהוֹצִיאָה מַשָּׂא, עָבַר הוּא עָלֶיהָ עַל מִצְוַת עֲשֵׂה שֶׁל תּוֹרָה. וְאַף דָּבָר שֶׁהוּא לָהּ לְנוֹי, מִכָּל מָקוֹם הֲוֵי מַשָֹא. אֲבָל דָּבָר שֶׁהוּא לִרְפוּאָתָהּ, כְּמוֹ אֶגֶד[תַּחְבֹּשֶת] שֶׁעַל הַמַּכָּה מֻתֶּרֶת לָצֵאת בּוֹ. וְכֵן דָּבָר שֶׁהִיא צְרִיכָה לִשְׁמִירָתָהּ, הֲוֵי לָהּ כְּמוֹ מַלְבּוּשׁ לָאָדָם וּמֻתֶּרֶת לָצֵאת בּוֹ. וְאַךְ מַה שֶּׁהוּא רַק לִשְׁמִירָה יְתֵרָה, אָסוּר. וְכֵן דָּבָר שֶׁבְּהֵמָה זוֹ אֵינָהּ מִשְׁתַּמֶרֶת בָּזֶה, אַף עַל פִּי שֶׁבְּהֵמָה אַחֶרֶת מִשְׁתַּמֶרֶת בּוֹ, הֲוֵי לָהּ לְגַבֵּי בְּהֵמָה זוֹ מַשָּׂא וְאָסוּר.
It is written [in the Torah]:1Exodus 23: 12. "…so that your ox and donkey may rest…" Here the Torah commands us that the animals of a Jew must also rest [on Shabbos]; and not only cattle, but all animals. Therefore, it is forbidden to allow your animal to carry out a load, because even if the animal went out on its own, into the public domain carrying a burden, you have violated a positive command of the Torah.2Although the animal may be permitted to do a melachah (“work”) of its own volition, such as eating growing grass, you must not allow it to do a melachah which is not of its own will, such as carrying a load. [v. Mishnah Berurah 305: 77] Even if the object [the animal is carrying] is ornamental, nevertheless, it is [considered] a burden. However, if the purpose of the object is to heal [the animal], such as a bandage on a wound, the animal may go out with it. Similarly, anything the animal requires for its safekeeping,3An identification collar may not be worn because it is only an indirect safeguard for the animal. [Aruch Hashulchan 305: 5] Rabbi S. Z. Auerbach zatzal, disagrees and permits it, for it ultimately benefits the animal. is considered like a garment for a human being and the animal may go out with it. But something which is for excessive safeguarding4See next paragraph. is forbidden.5Concerning the wearing of objects to ward off an עַיִן הָרַע (evil eye) or to prevent damaging others, such as a muzzle, see Shulchan Aruch 305: 11, 17 and below paragraph 7. Similarly, an object that this [particular] animal cannot be safeguarded with, though another animal can be safeguarded with it, is considered a burden for this animal and is forbidden.
תנא דבי אליהו כל השונה הלכות בכל יום מובטח לו שהוא בן העולם הבא שנאמר הליכות עולם לו אל תקרי הליכות אלא הלכות (נדה ע"ג.)