Iām 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.
Thatās 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. Iām not really sure why thereās a large enough demand for 1ā³-and-larger piping that every fabric store seems to carry the cording, but Iām not going to complain.
Piping cord comes in all sizes, from much smaller than the kind I used in the panther (1/4ā³, I think) all the way up to larger than the kind itās holding (1ā³) which Iāve used for cosplay tails and will use for the larger-than-life Mew. But if you canāt 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 āgrabbyā the inside of your fabric is and how long the piece youāre stuffing is, you might need to sew a casing for the piping out of satin(y) lining fabric to help slide it in. Thatās what Iāve done for the Mew tail, in the foreground there. It doesnāt 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 ā thatās plain olā cellophane tape. Wrap a piece around the cord where youāre going to cut it, then cut right through the middle of it. Donāt take the tape off until youāre ready to use the cording, or the end will shred. If Iām 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 ā itās just loose cotton batting, after all. But itās far, far better than fiberfill. Go forth, and make non-lumpy plushies!