"J" is for "Jawbreaker"

Jawbreaker

No. Enc.:  1d4 (1d4)
Alignment:  Neutral
Movement:  120' (40')
     —Swim:  45' (15')
Armor Class:  3
Hit Dice:  4
Attacks:  1 (bite)
Damage:  2d6+1
Save:  L2
Morale:  11
Hoard Class:  None
XP:  135

Jawbreakers are armored, 4'-5' fish that inhabit murky swamps, lakes, and rivers.  They are slow and sluggish in the water, and drift lazily beneath the surface...

...until prey nears the shoreline.  At that point, a jawbreaker shifts its back fins into nimble legs with clawed feet, and springs onto the land, Surprising on a 1-5 on 1d6.  It then rushes after prey with gnashing, bone-snapping teeth.

If a jawbreaker attacks a pack animal or mount with a successful To Hit roll of 17-20, it snaps the target's leg, resulting in crippling immobility.  PCs struck on a 19-20 with such an attack suffer a loss of -4 to DEX and a quartered movement rate until the injury heals in 1d4 months.

Mutations:  Aberrant Form ("Transforming Fins")