What does this idiom mean: "She gets on her high horse"?

Test No. [color=blue]incompl/inter-56 “High and Low”, question 8

It’s always the same when you break one of the rules of the office, she gets on her high and starts shouting at you.

(a) camel
(b) pony
© donkey
(d) horse

Test No. [color=blue]incompl/inter-56 “High and Low”, answer 8

It’s always the same when you break one of the rules of the office, she gets on her high horse and starts shouting at you.

Correct answer: (d) horse

Your answer was: n/a
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What does it mean?
she gets on her hight hourse

It means to be [color=blue]very arrogant.