Dungeon Snails

Often found in damp dungeons and sometimes they smother them in plague swarms especially hiding from droughts.  Baby ones are dangerous in swarms but weak alone. Bug ones are excellent for evil mounts and snail jousting has caught on in places where slow sports are a craze. They eat anything and are edible. Hungry adventures may eat them but they may have revolting parasites like giant wasp or carrion worm larvae. They are immune to moulds, slimes, oozes,puddings and like to eat them. They leave slimy silver trails and shit everywhere. They are nocturnal and use vibration, scent and crude 360 degree vision with stalks fully erect. Shell can be a blindspot in many cases.  Most prefer moist conditions. Drought or ice makes most types hibernate often in burrows or group nests. The large ones make slow but sure beast of burden that carry huge loads up sheet surfaces. If conditions are dry they may burrow and hibernate for years. All hate salt, a handful causes a d4 damage, pound d6, bucket d8. Brine or salty water drives most types away but only causes 1hp damage a bucket or more..

Baby d4 HD AV as leather Att d4 bite MOV 3
Adult 1-2 d8 HD AC as chain Att d6 bite MOV 6
Giant 4-8 d8 HD AC as plate Att 2d4 bite MOV 3

1 Dungeon snails can track prey by scent and can kill silently from ambush especially on the sleeping
2 Chaos Snails  Evil and have mutations, fearless to suicidal, eating them may cause mutations
3 Water Snails
Operate from underwater water mostly, sea ones are salt tolerant. Some can carry a passenger inside per HD (d4 HD only a small creature)
4 Fire Snails Immune to natural or magical heat or fire plus their touch ignites flammables with lava like hot slime. Water harms these snails as salt does most types. Cold does +1 per dice versus them
5 War Snails These spiky shelled snails have a horned charge attack instead of bite, very obedient fearless mounts with a rider who they always follow
6 Slugs
Naked snails have a lower profile and less armour -3 worse, better at squeezing and burrowing
7 Blood Snails
Bite grabs hold with first hit then suck blood each round until victim dead through hideous toothed oral sphincter, Leave mummified husks after draining prey, they also eat these remains too
8 Ice snails shells made of ice crystal immune to natural and magical cold. Heat or fire does +1 per dice versus them
9 Wizard Snails Detect magic and smells of wizards, once eat a wizard brains they become more cunning and can cast spells as a d4 level wizard. Can speak and converse once can cast spells
10 Phase Snails Can retract into their mutidimentional fractal shells +4AC and can only be hit by magic if hide in shell, effectively invisible
11 Necro Snails 
Infected partially gnawed corpses arise as zombies in 24 hours. Control one per HD of snail which defend and serve thew snail. Small ones use zombies as mounts
12 Brain Snail
Can charm a victim per HD with addictive slime works on bite or touch on non shell snail. The first thrall makes them sentient with 7 INT & CHA. As they gain thralls they become smarter (+1 per thrall).  Small ones use thralls as mounts. They need new thralls as they feed of victims neural and brain fluid eating a DEX or INT point a day per HD. They become Machiavellian schemers and are often start cults or gangs using thralls as leaders

Common Mutations
1 Extra Head +1 Attack with head, extra head per extra mutation
2 Explode on death d4 per HD within 20 foot or save for half, (d4 HD only a 10 foot radius blast)
3 Acid Spit once a day per HD, damage as bite, 1 range per HD, damages items
4 Hyper speed Mov 12 as a human!
5 Regenerate 1hp/round unless fire, acid or salt used to harm it
6 Teleport 3 range once per day per HD
7 Breath Fire D3 +1/HD, 3 range cone (1 range for d4HD)
8 Beserker if zero keeps fighting till reaches -10, and +2 hit and damage
9 Invisibility turn per HD per day (d4 HD snails only a d4 rounds per day
10 Iron Shell +2 AC
11 Huge Teeth, bump up dice to next higher type
12 Intelligent can talk, long lived

Yes you can have a chaos necro dungeon snail with regeneration and two heads that explodes on death. Yes they hang out with flail snails.