Grammar Check for a Sentence About Calves and Oryx Horns for a 9-Year-Old

The sentence you wrote is mostly clear, but it can be improved a little bit for better grammar and clarity.

Your sentence:
"When a calf's horns grow long, it can protect itself like the other oryx in the herd."

Suggestions:

• Use "others" instead of "other" because you're talking about more than one oryx.
• "Itself" is correct because the calf is protecting itself.
• You might also say "like the other oryxes in the herd" or simply "like the other oryx in the herd" (since "oryx" can be singular or plural).

A corrected sentence could be:

"When a calf's horns grow long, it can protect itself like the other oryx in the herd." (If you think "oryx" is singular and plural.)

Or:

"When a calf's horns grow long, it can protect itself like the other oryxes in the herd." (If you use "oryxes" as the plural.)

Both are okay, but most commonly "oryx" is used for both singular and plural, so your original sentence is actually fine with just changing "other" to "others":

"When a calf's horns grow long, it can protect itself like the others in the herd."

But if you want to keep the word "oryx," keep it singular or plural depending on how you use it.