I was thinking about some of the cool color effects out there and I just whipped this off of the top of my head.
creature
[CREATURE:SAND_SNAKES]
[NAME:sand snakes:groups of sand snakes:sand snake]
[TILE:247][COLOR:16:0:0]
[LARGE_ROAMING][LARGE_PREDATOR]
[POPULATION_NUMBER:25:50]
[CLUSTER_NUMBER:5:10]
[GRASS_TRAMPLE:42][BLOODTYPE:G]
[ALL_ACTIVE][EXTRAVISION][NOT_BUTCHERABLE]
[NO_PAIN][NO_FEAR][STOUT]
[PREFSTRING:hiding abilities]
[PREFSTRING:poison venom]
[PREFSTRING:chameleon skin]
[EXTRACT:sand snake venom:7:0:1]
[EXTRACT_VALUE:0]
[EXTRACT_PARALYZE]
[BODY:SERPENTS:2EYES:2LUNGS:HEART:GUTS:ORGANS:THROAT:NECK:SPINE:BRAIN:FANGS]
[SIZE:8][FAT:0]
[MAXAGE:25:50]
[liTTERSIZE:3:5]
[CHILD:1][CHILDNAME:sand snakelets:groups of sand snakelets]
[ATTACK:MAIN:BYTYPE:MOUTH:bite repeatedly:bite repeatedly into:2:5:GORE][ATTACKFLAG_CANLATCH][SPECIALATTACK_INJECT_EXTRACT:250:500]
[BIOME:DESERT_SAND]
[STANDARD_FLESH]
[COLDDAM_POINT:10000]
[SWIMS_LEARNED][SWIM_SPEED:5000]
body
[BODY:SERPENTS]
[BP:UB:upper body][UPPERBODY]
[BP:MB:mid body][CON:UB][LOWERBODY]
[BP:LB:lower body][CON:MB][GRASP]
[BP:TAIL:tail][CON:LB][STANCE]
[BP:HD:head][CON:UB][HEAD]
[BP:UB2:upper body][UPPERBODY]
[BP:MB2:mid body][CON:UB2][LOWERBODY]
[BP:LB2:lower body][CON:MB2][GRASP]
[BP:TAIL2:tail][CON:LB2][STANCE]
[BP:HD2:head][CON:UB2][HEAD]
[BP:UB3:upper body][UPPERBODY]
[BP:MB3:mid body][CON:UB3][LOWERBODY]
[BP:LB3:lower body][CON:MB3][GRASP]
[BP:TAIL3:tail][CON:LB3][STANCE]
[BP:HD3:head][CON:UB3][HEAD]
[BODY:FANGS]
[BP:FANGS:fangs][CONTYPE:MOUTH][SMALL][EMBEDDED][SKELETON]
This should create a tile of snakes that appears only in warm sand deserts, yet chameleonizes itself to look the exact same color as the tile beneath it. As its tile is the ≈, the same tile as sand, this should make it nigh invisible to the player until his dwarves start running away in fear. It has paralyzing venom, and appears in groups of five to ten, with large amounts showing up on your map at a time and overall. It also is a large predator, with a size of 8, meaning it should also attack dwarves, humans, and elves with no hesitation. I am a little worried about the body, though, as it is essentially creating three bodies in a single tile and will probably glitch horribly/create weird effects when tried.