Both a giant gator and a giant croc. The crocodile is at least based on it's primitive ancestors. I admit to knowing next to nothing about alligator ancestry.
Place these in creature_large_temperate.txt
Beware, these guys are really really dangerous.
[CREATURE:ALLIGATOR_GIANT]
[NAME:giant alligator:giant alligators:giant alligator]
[TILE:'A'][COLOR:2:0:1]
[MODVALUE:20]
[POPULATION_NUMBER:2:3]
[CLUSTER_NUMBER:1:1]
[LARGE_PREDATOR][SAVAGE]
[LARGE_ROAMING][DIFFICULTY:4]
[AMPHIBIOUS]
[GENPOWER:5]
[PETVALUE:2000]
[GRASSTRAMPLE:20]
[CARNIVORE][NATURAL][PET_EXOTIC][MOUNT_EXOTIC]
[PREFSTRING:size]
[BODY:QUADRUPED:TAIL:2EYES:NOSE:2LUNGS:HEART:GUTS:ORGANS:THROAT:NECK:SPINE:BRAIN]
[SIZE:14]
[MAXAGE:60:200]
[ATTACK:MAIN:BYTYPE:HEAD:bite:bites:1:6:GORE][ATTACKFLAG_CANLATCH]
[CHILD:12][CHILDNAME:giant alligator hatchling:giant alligator hatchlings]
[FAT:6]
[ALL_ACTIVE]
[BIOME_SWAMP_TEMPERATE_FRESHWATER]
[BIOME_MARSH_TEMPERATE_FRESHWATER]
[BIOME_SWAMP_TROPICAL_FRESHWATER]
[BIOME_MARSH_TROPICAL_FRESHWATER]
[CREATURE:CROCODILE_GIANT]
[NAME:giant crocodile:giant crocodiles:giant crocodile]
[TILE:'C'][COLOR:7:0:0]
[MODVALUE:20]
[POPULATION_NUMBER:2:3]
[CLUSTER_NUMBER:1:1]
[LARGE_PREDATOR][SAVAGE]
[LARGE_ROAMING][DIFFICULTY:5]
[AMPHIBIOUS]
[GENPOWER:5]
[PETVALUE:2000]
[GRASSTRAMPLE:20]
[CARNIVORE][NATURAL][PET_EXOTIC][MOUNT_EXOTIC]
[PREFSTRING:size]
[BODY:QUADRUPED:TAIL:2EYES:NOSE:2LUNGS:HEART:GUTS:ORGANS:THROAT:NECK:SPINE:BRAIN]
[SIZE:16]
[MAXAGE:600:1200]
[ATTACK:MAIN:BYTYPE:HEAD:bite:bites:1:8:GORE][ATTACKFLAG_CANLATCH]
[CHILD:12][CHILDNAME:giant crocodile hatchling:giant crocodile hatchlings]
[FAT:8]
[ALL_ACTIVE]
[BIOME_SWAMP_TEMPERATE_SALTWATER]
[BIOME_MARSH_TEMPERATE_SALTWATER]
[BIOME_SWAMP_TROPICAL_SALTWATER]
[BIOME_MARSH_TROPICAL_SALTWATER]
[BIOME_SWAMP_MANGROVE]