Thanks for all the nice comments!
I added upper and lower palettes of foam to the head to help define the shape of the muzzle and provide a mouth interior.
Covering with fleece:
I used polar fleece fabric to cover the Blue Whale. You can also use faux fur, or any other fabric you want. I prefer fleece because it is so pleasant to handle, and makes relatively invisible seams. The Zebra is "plush" short pile synthetic faux fur, which is also pleasant to use.
We learned on the Blue Whale that sometimes the "facets" of the foam shapes show through as hard lines under the fleece, so to avoid this and make a smoother shape, cover the costume first with a layer of poly batting. I didn't have time to do it on the Whale, so he's a little lumpy.
If you're making a small shape and/or you can tell the facets won't show through, you might not have to do this.
I used the palette patterns to cut shapes from pink fleece for the mouth interior, and sewed them to the foam with matching thread. For a costume in which the wearer sees through the mouth, you need to make sure there's enough of an opening at the back of the mouth to see through, and cover it with dark netting so the wearer isn't visible.
Sometimes it's better to finish the mouth interior last, so you can get inside the head easily.
Then drape the costume with fleece, and pin it in place. You can make a muslin pattern if you want, but I find it easier and faster to drape directly on the object, cutting and fitting the fabric on the shapes. This can be wasteful of fleece, though. I try to conceal seams in anatomical features, if possible. Don't pull the fleece too tight around the shapes because it can distort the foam, just smooth it over the contours.
I leave extra around the mouth to fit around the lips.
Carefully pin down and trim. You'll get the smoothest seam if you trim the fabric so the edges just meet.
Sew seams using small even stitches. I use automotive upholstery thread for most costume projects. I'm using regular thread for this little guy.
Depending on how fussy you are, you can get the seam to almost disappear by teasing the fibers with a pin.
Here's the head with the basic covering. I made a tongue from fleece also:
More in the next day or two about detailing.