Buttercup

Buttercup is one of the toys owned by Bonnie that appears in Toy Story 3. He is a white unicorn with a golden horn, mane, tail, hooves, and two pink hearts on his nostrils. And although he looks exactly as if he were a female unicorn, he is a male. He is voiced by Jeff Garlin.

In the end credits, he is shown becoming close friends with Hamm, as the two are seen standing side-by-side when Woody reads his friends a message from Sunnyside Daycare written by Ken, when they watch an Alien and Mr. Pricklepants perform their rendition of Shakespeare's Romeo and Juliet, and when they enjoy watching Buzz and Jessie perform a paso doble.

Hawaiian Vacation
Buttercup appears in the short. When Barbie and Ken go horseback riding Ken jumps on Buttercup and tells him to go, but Buttercup replies by saying "Sure, now get off me".

Toy Description
From Official Website:

"Saddle up for fun and let Buttercup lead your child away on a magical adventure! This cuddly unicorn features velvety-soft, snow-covered fur with sparkly gold and pink accents. He sports a signature mythical golden horn and a fun-to-comb mane and tail. Buttercup's durably plastic eyes are both charming and scratch-resistant. Hypo-allergenic. Ages 3 and up."

Trivia

 * At one point in the movie, Buttercup calls Mr. Pricklepants "Baron Von Shush". This is because Mr. Pricklepants kept cutting Woody off by shushing him. To which Buttercup replies "The guy's just asking a question!"
 * Bttercup was originally going to be part of Lotso's gang as seen in some Toy Story 3 concept sketches.