I once had an idea for a Clan-backstory based class that was kind of horrible but the concept was entertaining.
The Swallowed Ones
A strange semi-cult of treasure hunters and fanatics. They seek out the largest of monsters that usually have been the downfall of legendary figures (Party-killers) and attempt to retrieve the historical artifacts and magical items that tend to be… left behind. For them the story of "Old Man swallowed by the whale" wasn't a parable, it was their elder.
Due to their nature, the Swallowed One slowly gains Resistance to acid throughout their career starting at level 2 (culminating at Immunity much later).
The more fanatical and religious minded revere the Tarrasque as like to a God. The vast amount of epic or legendary equipment, items, armor, weapons, gems, metals, etc. that could survive it's stomach unable to be digested,
Lower levels are referred to as "Muckers" for their habit of sorting through monster droppings for anything that survived digestion. Think of the coins hapless halflings or kobolds may have had on them when swallowed whole. Luckily for them, and anyone they go near, the two starting abilities
"Breathless" (supernatural ability) : they no longer need to breathe except to smell or speak.
"Neutralize" (spell-like ability) : Cleans the Swallowed One, equipment, and all non-living material (treasure) they are holding of residual acid, smell, and filth. This can be used once per day with an additional use per day every five levels (6th, 11th, 16th, etc.).
At 4th level, the Swallowed Ones earn their name with a new ability -> "Gastronaut" : when being attacked by a creature more than 2 size categories greater using a bite attack they make a DC 20 reflex check to avoid all damage and be completely swallowed by the creature.
At 14th level they gain "Escape-hatch" (supernatural ability) 1/day. Usable only INSIDE a monster (dead or alive). This teleports them to 100ft past the physical sensory range of the creature they are escaping.