Hello! I checked your four sentences about a kiang. All of them are mostly correct. Here are small fixes and easy explanations so they sound clear.
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Original: A kiang is a wild donkey that lives in the mountains and casually skips along cliffs.
Better: A kiang is a wild donkey that lives in the mountains and casually skips along the cliffs.
Why: Adding "the" before "cliffs" makes the sentence sound a little more natural, but the original was okay.
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Original: A kiang is a wild donkey that lives in the mountains and hops along cliff edges.
Better: This sentence is fine as it is.
Why: It is clear and correct.
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Original: A kiang is a wild mountain donkey that hops along cliff edges.
Better: This sentence is also fine.
Why: "Wild mountain donkey" is a short way to say the same thing as the longer version.
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Original: A kiang is a wild donkey that hops along cliff edges high in the mountains.
Better: A kiang is a wild donkey that lives high in the mountains and hops along cliff edges.
Why: Putting "high in the mountains" next to "lives" makes the meaning clearer: it tells where the kiang lives.
Short answer: All four were okay. I gave tiny changes to make two of them sound a bit clearer. Great job!