Barbie Gallery numbers 15 Fashion Fairytale, 17 Stacey, and 18 Merliah Summers in Barbie in a Mermaid Tale. All of these Barbie pictures have incredibly thick ponytails. To get this fullness in a ponytail, I have a little trick: use a thread spool.
Begin gathering the hair into a ponytail. For number 15, make it a very high ponytail at the top of the head. Put an empty spool at the bottom of the ponytail and let the hair slide around it to conceal it.