The idiom “害群之马” in English can be translated as “a black sheep” or “a troublemaker.” Both expressions convey the meaning of someone who is detrimental to the group or causes problems within it.

“A black sheep” is a common English idiom that refers to someone who is considered different or out of place within a group. This person may have negative traits or behaviors that set them apart from others, making them a burden or a source of trouble. For example, if a person is always causing trouble at work or is disliked by their peers, they might be described as “a black sheep.”

On the other hand, “a troublemaker” is a more direct translation of the Chinese idiom. It refers to someone who deliberately causes trouble or disrupts the harmony of a group. This person may be seen as a troublemaking individual who consistently brings negative attention to themselves and others around them.

Both expressions can be used in various contexts to describe someone who is detrimental to a group or organization. Here are a few examples:

1. If a team member is always late for meetings and causes disruptions, you might say, “He’s a real black sheep in our team.”

2. A student who consistently skips class and causes disturbances might be described as, “She’s a troublemaker in our school.”

3. If a colleague is always spreading rumors and causing conflicts, you might say, “He’s a real害群之马 in the office.”

It’s important to note that the choice between “a black sheep” and “a troublemaker” may depend on the context and the specific traits or behaviors of the person being described. In some cases, you might even use both expressions to emphasize the negative impact the person has on the group.