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

[Pupil]

Test No. 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
(c) donkey
(d) horse

Test No. 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
_________________________

What does it mean?
she gets on her hight hourse
_________________
the more we learn
the less we know

[Alan]

It means to be very arrogant.
_________________
English as a Second Language
You can read my ESL story Reflections