The last big piece you need to crochet is Billy's mane. Grab the blue yarn and make a magic loop with 4 single crochet stitches in it.
Round 1: 4 sc in magic loop (4)
Round 2: inc x 4 (8)
Round 3: [sc, inc] x 4 (12)
Round 4-23: 12 sc (12)
Fasten off, leaving a long tail. Curl the closed end of the mane, and lightly stuff the rest of the mane. Do not put stuffing in the curled part of the mane.
Then whip stitch the opening closed, and attach the mane immediately behind the horn. Watch this video to see how to do this: