Final answer:
Using an example from the past to justify a present injustice can be an appeal to tradition, where old practices are deemed right solely due to their historical use, or a moral equivalence fallacy, where past and present actions are wrongly equated in terms of their ethical implications.
Step-by-step explanation:
The type of logical fallacy you're describing, where an example from the past is used to justify present injustice, can be related to a number of different fallacies depending on the context. The most common logical fallacies that might apply here are the appeal to tradition and the moral equivalence fallacy.
An appeal to tradition is a fallacy wherein something is considered better or acceptable simply because it is older or has been done for a long time. For instance, the argument that a discriminatory practice is acceptable because it has historically been the norm falls into this category of logical fallacy.
The moral equivalence fallacy is making a comparison that exaggerates the similarities of two very different actions, often implying that both are equally ethical or unethical. If someone justifies current unethical behavior by comparing it to past events, saying the past events were similar or just as bad, they may be committing this fallacy.