It means to be angry on behalf of others.
This “others” does not refer to a related person, but has the nuance of a certain distance, and is not used for family members or relatives.
Leruns are usually calm and friendly to humans, and even hardly react when a violent person kicks them, but when Lemuns, their symbiotic counterparts, are attacked, they turn violent.
Translation note
Lerun: The largest herbivore on land. They have been raised since ancient times because of the high-quality fur they produce. Leruns and Lemuns are well known for their extremely close symbiotic relationship. In return for a Lemun taking care of Lerun juveniles, Leruns fight with all their might for them when a Lemun is attacked by predators such as Valood. There are many myths, folklore, and proverbs related to this relationship. Leruns are also called “guardians” since they can protect humans just like how they protect Lemuns if a trusting relationship is established.
Lemun: An animal with a position comparable to a dog or cat on Earth. It is extremely friendly and intelligent, and therefore many people tend to live with it as a family. It is well known for its exceptionally close symbiotic relationship with Leruns, the largest herbivore on land. The Lemun takes care of Lerun juveniles, and in return, the Lerun becomes violent and fights with all its might when a Lemun is attacked by predators.
Reference
“Dictionary of Proverbs by Biological Species ‘Lerun’” (Direlle Aarti)