Pattern:
Knit up a stockinette stitch triangle & bind off leaving a VERY
LONG yarn tail.
I use worsted weight wool on two #8 straight needles & cast on 3 sts, then work in st st, adding simple increases (knit into the front & back of st) at the beginning & end of each knit row until triangle/mouse is desired length.
Use the yarn tail to stitch up the short open side, form 3 large "loops" of yarn (these will become the tail), sew a few tight stitches to secure "tail", then use the rest of the sewing/yarn end to stitch up the last open side (leaving a small hole for stuffing), form tiny loops on each side of the head end for "ears," then secure & weave in yarn end very well. For a natural-looking mouse, be sure to round off the "rear" seam. Make it a nice, round curve!
Then finish the tail: braid the three long yarn loops, tie a couple of very tight knots near the end, and clip the ends to form a little "fringe" at the very tip. Next, "full/felt" the unstuffed mice by tossing them in the washer & dryer. Stuff, adding 1/2 tsp of catnip to very center of stuffing, if desired. Add a few tightly-secured embroidery-floss whiskers & eyes.