Felt Dolphin toy pattern, nice and simple, perhaps add bells, lavender or a rattle inside? @scottibe this is a little like the cat rattle you made. The site also has an amazing giraffe and zebra plushie pattern.
Celebrate National Dolphin Day with dolphin knitting patterns - Dolphin toy, dolphin dish cloth (free!), dolphin hat, and dolphin scarf (free!) http://intheloopknitting.com/sea-animal-knitting-patterns/