"Excited Zebra!" joke

There was this zebra who had lived her entire life in a zoo and was getting on a bit so the zoo keeper decided as a treat that she could spend her final years in bliss on a farm.
The zebra was so excited, she got out of the horse float to see this huge space with green grass and hill and trees and all these strange animals. She saw a big fat weird looking brown thing and ran up to it all excited, "Hi! I'm a zebra. What are you?"
"I'm a cow," said the cow.
"Right, right, what do you do?"
"I make milk for the farmer."
"Cool."
The zebra then saw this funny looking little white thing and ran over to it. "Hi, I'm a zebra. What are you?"
"I'm a chicken," said the chicken.
"Oh, right, what do you do?"
"I make eggs for the farmer."
"Right, great, see you round."
Then the zebra saw this very handsome beast that looked almost exactly like her without the stripes.
She ran over to it and said, "Hi, I'm a zebra. What are you?"
"I am a horse," said the horse.
"Wow," said the zebra. "What do you do?"
"Take off your pajamas, darling, and I'll show you."