Costume Ideas by Character:

Cats:

Or

DIY http://potpourrimommy.blogspot.com/2012/01/cat-in-hat.html

Horton:

Jojo

Hairstyles:

Mayzie

Gertrude:

Whos:

Big Bow Ties, Polka Dots, Stripes, Striped socks

Mr. Mayor and Mrs. Mayor are more proper with maybe a hat for Mrs. Mayor and a bowtie and jacket for Mr. Mayor

Young Kangaroo:

Sour Kangaroo

Vlad Vladikof:

Head Mask like Lion King (Courtney)

Wings:

Wickershams:

Jungle Citizens:Darker base of grey or brown, black with white. Maybe animal print.