Rava said: An ox that gored a maidservant and her offspring came out [she miscarried] - the owner pays the value of the offspring. What is the reason? It is a pregnant she-donkey that he damaged, for Scripture says, "Stay here with the donkey" (Genesis 22:5) [read as] "the people who are like a donkey."
Rava on an Ox That Gores a Maidservant and Her Unborn
Yalkut Shimoni on Torah 100:3
אָמַר רָבָא, שׁוֹר שֶׁנָּגַח אֶת הַשִּׁפְחָה, וְיָצְאוּ יְלָדֶיהָ, מְשַׁלֵּם דְּמֵי וְלָדוֹת, מַאי טַעְמָא חַמְרָא מְעֻבַּרְתָּא הוּא דְאַזֵּקְתְּ דְּאָמַר קְרָא שְׁבוּ לָכֶם פֹּה עִם הַחֲמוֹר, עַם הַדּוֹמֶה לַחֲמוֹר.