A Pygmy Puff is a miniature puffskein. We are introduced to Pygmy Puffs in Harry Potter and the Half-Blood Prince when Ginny Weasley buys one from Weasleys' Wizard Wheezes. They are pink and purple miniature puffskeins and, according to Fred and George, are very popular.
Dear Messrs. Weasley:
It was a pleasure to discover your shop and your line of various Wizard Wheezes, including my new dear pygmy puff, Marius, though it seems he has developed some sort of condition.
As you recall he was a lovely purple when I purchased him and took him home. Recently, he has developed some rather odd colorations that cannot be explained (photo enclosed) Sincerely
Dear Ms. Dragonsinger:
It is a pleasure to hear from you and we're glad you enjoyed your visit to our shop. As for Marius' condition, it seems he's become goth in his tastes and is otherwise fine and healthy.
Sincerely Fred and George Weasley.
PS: You might find him listening to muggle performers, Metallica and Voltaire.