I’m gonna let you in on a little trade secret. Fiberfill is bad for plushie tails. Both Mew and the black panther have long skinny tails (though I managed to photograph the panther WITHOUT ITS TAIL IN VIEW). If I stuffed that kind of tail with fiberfill, it would be a lumpy string-of-beads sort of tail.
That’s not the secret.
The secret is to use a product made for the purpose: filler cord. This comes in all sizes, in cotton or polyester, and is made for inserting in piping. I’m not really sure why there’s a large enough demand for 1″-and-larger piping that every fabric store seems to carry the cording, but I’m not going to complain.
Piping cord comes in all sizes, from much smaller than the kind I used in the panther (1/4″, I think) all the way up to larger than the kind it’s holding (1″) which I’ve used for cosplay tails and will use for the larger-than-life Mew. But if you can’t find the size you need, an alternative is thin quilt batting. Cut it to length and roll it up (or roll it around too-thin piping cord) until you get to the size you need.
Depending on how “grabby” the inside of your fabric is and how long the piece you’re stuffing is, you might need to sew a casing for the piping out of satin(y) lining fabric to help slide it in. That’s what I’ve done for the Mew tail, in the foreground there. It doesn’t need to be turned; the whole thing will slide right inside the actual faux-fur tail.
You may also notice that it looks like there are shoelace aglets on the ends of the piping cord – that’s plain ol’ cellophane tape. Wrap a piece around the cord where you’re going to cut it, then cut right through the middle of it. Don’t take the tape off until you’re ready to use the cording, or the end will shred. If I’m not enclosing the cording in satin, I will wrap the end in thread to keep it from shredding during final insertion.
Piping cord can, with a lot of (ab)use, bunch up some — it’s just loose cotton batting, after all. But it’s far, far better than fiberfill. Go forth, and make non-lumpy plushies!