If we see an animal belonging to another Jew – or another Jew himself – collapsed beneath a heavy load, we have an obligation to help out. This involves unpacking the load and re-packing it properly. Here, we actually have a negative mitzvah: we are prohibited from passing by without offering our assistance. The positive mitzvah is found way back in parshas Mishpatim (Mitzvah #80) and is stated specifically about helping even someone we don’t like.
The reason for this mitzvah is what we said in Mitzvah #80 – helping out in such a situation helps to cultivate in us the trait of kindness towards both man and beast.
This mitzvah says “your brother,” limiting it to other Jews. This is because there is a general rule of reciprocity in mitzvos. For example, Jews are not allowed to steal from non-Jews; non-Jews are likewise prohibited (by the seven universal or “Noachide” laws) from stealing from Jews (or from one another). Since there is no obligation on non-Jews to assist an animal fallen under its load, there is no specific obligation to help them in such a case. However, if the animal is in distress because of the situation, one would be obligated by the Torah to assist because of tzaar ba’alei chayim – to prevent animals from unnecessary suffering. Even if the animal is not suffering, we are required by rabbinic law to assist non-Jews in such a case in order to avoid resentment. (See Talmud Baba Metzia 32b.)
This mitzvah applies in all times and places. It is discussed in the Talmud in tractate Baba Metzia on pages 32a-33a and codified in the Mishneh Torah in the thirteenth chapter of Hilchos Rotzeiach. This mitzvah is #270 of the 365 negative mitzvos in the Rambam’s Sefer HaMitzvos and #183 of the 194 negative mitzvos that can be observed today as listed in the Sefer HaMitzvos HaKatzar of the Chofetz Chaim.