Notes:

• Piece is worked from tail forward to face.
  
• All knitting is in the round.  The pictures show the magic loop method using a circular needle, but double points would be fine.  Just pull the yarn tight between needles to prevent gaps.
• The fins and ears are shown as knitted directly onto the body by picking up (and knitting) stitches.  If you choose this method, you may find it easier to pick up the stitches with a crochet hook and then transferring to the needle.  You can also knit the appendages separately and then attach using small whipstitches.  Just cast on the number of stitches that would have been picked up.
• Gauge must be very tight.  No specific gauge is recommended, but use needles with your yarn that will make a very solid fabric so the catfish will keep its shape and the fiberfil won't show.
• I used left- and right-leaning lifted increases, but any increase can be used.  It will, of course, affect the finished appearance.
  
• The catfish pictured is about 6" (15cm).

Materials:

• Worsted weight yarn.  I used Berroco Comfort, but feel free to substitute.
• Needles required for stiff gauge with chosen yarn.
• 2 safety eyes (eyes shown are 12mm).  Cut felt or embroidery thread can be used instead.
• Fiberfil stuffing
• White clear-drying craft glue
• Plastic pellets (optional)
• Black yarn for face embroidery

Special abbreviations:

• RL - Right-leaning lifted increase (knit st, then knit through st 2 st below)
  
• LL - Left-leaning lifted increase (knit through st below, then knit st)

Body:

Figure 1: Joining tail pieces

Note: You will work rows 1-10 twice, making two separate tail points.  Cut the yarn and move to temporary needles after the first part only.  Leave the second fin piece on the needles and do not cut yarn.  You will knit across the front and back of the two pieces to join.
CO 4 stitches
Row 1 (and all odd rows): knit
Row 2: K1, LL, RL, k1 (6 st)
Row 4: ssk, LL, RL, k2tog (6)
Row 6: k2, LL, RL, k2 (8)
Row 8: ssk, k1, LL, RL, k1, k2tog (8)
Row 10: k3, LL, RL, k3 (10)
For first tail part, place stitches on two dpns or spare circular needles, cut yarn.
Work rows 1-10 again (second piece).  For second part, do not cut yarn.

Row 11: (See Figure 1)  Knit across next 5 stitches of live piece on the needles.  Making sure to mirror the piece direction, knit across 5 stitches on the other piece, then across the back of that piece and finally across the back of the first piece.  (20 st, 10 on each side)
Row 12-15: Knit plain.
Row 16: (ssk, k6, k2tog) X 2 (16 st)
Row 18: (ssk, k1, RL, LL, k1, k2tog) X 2 (16)
Row 20: Knit plain
Lightly stuff the tail pieces here, while they're still easy to get to.

Starting body increases:
Row 22: (RL, k2, RL, LL, k2, LL) X 2 (24)
Row 24: (RL, k1, RL, k1, RL, k2, LL, k1, LL, k1, LL) X 2 (36)
Row 26: Knit plain.
Row 28: (RL, k4, RL, k6, LL, k4, LL) X 2 (44)
Rows 30-41: Knit plain.

Starting neck decreases:
Row 42: (ssk, k4, ssk, k6, k2tog, k4, k2tog) X 2 (36)
Row 44: (ssk, k3, ssk, k4, k2tog k3, k2tog) X 2 (28)
Pause here to stuff most of the body.  The piece tends to be a little front-heavy, so you may want to put some plastic stuffing pellets in a small plastic bag or fabric pouch and place these at the tail "opening" before adding fiberfill.

Starting head:
Row 46: (RL, k3, RL, k4, LL, k3, LL) X 2 (36)
Row 48: (RL, k4, RL, k6, LL, k4, LL) X 2 (44)
Row 50: k2tog, k10, ssk, k2tog, k12, ssk, k2tog, k10, ssk (38)
Row 52: k2tog, k8, ssk, k2tog, k10, ssk, k2tog, k8, ssk (32)
Row 54: k2tog, k6, ssk, k2tog, k8, ssk, k2tog, k6, ssk (26)
Row 56: k2tog, k4, ssk, k2tog, k6, ssk, k2tog, k4, ssk (20)
Row 58: k2tog, k2, ssk, k2tog, k4, ssk, k2tog, k2, ssk (14)
If using safety eyes, place them after row 59, while the opening is still large enough.
Stuff the rest of the body and most of the head here.
Row 60: k2tog, ssk, k2tog, k2, ssk, k2tog, ssk (8)
Cut yarn, thread yarn needle, and sew through these 8 stitches, removing the knitting needle(s).  BEFORE pulling closed, finish stuffing the head.  Then pull the opening closed.  (See Figure 2.)  Sew in yarn end and tail ends.

Dorsal fin:
Note: Pictures show construction by picking up stitches on the body.  Fins and ears can also be cast on and worked separately, then sewn on.
On one side (one half stitch over) of back midline, pick up 6 stitches over 7 rows from back to front, pick up one stitch over midline, pick up 6 more stitches half stitch from midline going front to back, then one more stitch over midline.  (14 st)
Row 1 and all odd rows through row 7: Knit.
Row 2: Knit.
Row 4: k4, k2tog, k1, ssk, k5 (12)
Row 6: k3, k2tog, k1, ssk, k4 (10)
Row 8: k2, k2tog, k1, ssk, k3 (8)
Row 9: k1, k2tog, k1, ssk, k2 (6)
Row 10: k2tog, k1, ssk, k1 (4)
Cut yarn and thread through needle.  Sew through 4 stitches on the needle and pull closed.  Sew in yarn ends.

Side fins:
Place side fins about 1/3 down from dorsal fin.  Pick up 5 stitches over 6 rows, working from front to back.  Pick up 1 stitch over horizontal stitch at back, the pick up 5 more stitches from front to pick, one full stitch over from first 5 stitches, then one more stitch at front end between two sides..
(12 stitches)
Row 1 and all odd rows: Knit.
Row 2: ssk, k7, k2tog, k1 (10)
Row 4: ssk, k1, LL, k1, RL, k1, k2tog, k1 (10)
Row 6: ssk, k5, k2tog, k1 (8)
Row 8: ssk, LL, k1, RL, k2tog, k1 (8)
Row 10: ssk, k3, k2tog, k1 (6)
Row 12: ssk, k1, k2tog, k1 (4)
Cut yarn and thread through needle.  Sew through 4 stitches on the needle and pull closed.  Sew in yarn ends.
Repeat on other side.

Ears:
Pick up 4 stitches from side-to-side over 4 stitches over the eyes.  Pick up 4 more stitches on row behind that for back of ear.  (See Figure 5.)  (8 stitches)
Rows 1-3: Knit.
Row 4: (ssk, k2tog) X 2 (4)
Cut yarn and thread through needle.  Sew through 4 stitches on the needle and pull closed.  Sew in yarn ends.
Repeat on other side.

Finishing:
Sew on nose and mouth with black yarn or embroidery thread.  Cut 3 6" (15cm) lengths of body yarn.  Place a dab of white craft glue on your thumb, then run each length of yarn through the glue, leaving a small length unglued at one end.  Allow to dry completely.  (Lay it on waxed paper.)  Thread the unglued end through a yarn needle, the pull halfway through the "snout" for whiskers.  Trim the ends to desired length.