I was reading and I found the next sentence:
And I had that question, What's the difference between that and saying
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First, as a basic rule, が and の are interchangeable in relative clauses (with a few restrictions). 動きの鈍った魔物 and 動きが鈍った魔物 are totally interchangeable, and 動きが鈍い魔物 and 動きの鈍い魔物 are totally interchangeable, too. See: How does the の work in 「日本人の知らない日本語」?
Then what's the difference between 動きが鈍った魔物 and 動きが鈍い魔物? Simply, the former refers to monsters that are usually quick but have temporarily slowed down for some reason, while the latter tends to refer to monsters that are always slow. In other words, in the former case, the sentence means those monsters are tired, injured, or under some weakening magic.