"S" is for "Stiltfin"

Stiltfin

No. Enc.:  1d4 (2d8)

Alignment:  Neutral
Movement:  150' (50')
      —Swim:  90' (30')
Armor Class:  6
Hit Dice:  3
Attacks:  1 (bite, or kick, or gore, or trample)
Damage:  1d4 or 1d6 or 2d4 or 3d4
Save:  L2
Morale:  8
Hoard Class:  None
XP:  80

Stiltfins are freshwater fish that reach lengths of 6'-9' long, and measure 3'-5' from pelvic fins to spines.  They are encountered in lakes, rivers, and even underground caverns.

While primarily aquatic, stiltfins can travel over land by elongating their fins (so as to attain total heights of 12'-15') for use as makeshift legs.  This allows them to not only reach fruits and leaves in tall treetops, but also chase down live prey.

During rainy seasons, large schools of stiltfins (containing from 1d00-3d00 members) migrate across the plains to spawn in isolated lagoons.  Such throngs of the graceful fish are a wondrous sight indeed, but they can leave swaths of devastation in their wake, stripping vegetation bare and devouring wildlife.  And settlements dread few things more than a stiltfin stampede (whose members naturally use the Trample maneuver).

Mutations:  Aberrant Form (Gills and Lungs), Aberrant Form (Locomotive Fins)