The Worm Inside

The old creature concept from 2014, with some updated lore:

A long standing ancient parasitic or  symbiotic relationship between a parasite worm and a lizard. It most  likely started out as a purely parasitic relation, only benefitting the  worm, but eventually grew into a more symbiotic one where both sides now  rely on the other for survival.

The  worm's digestive system has evolved to consume as wide a range of  nutrients as possible, which it shares with its host. In addition to  this the worm is immune to most toxins, which it can carry over to the  lizard.

The lizard's tough skin offers  a good amount of protection and a home for the worm. Its strong legs  provide mobility, and the claws can be used for defensive and offensive  reasons. In emergency situations, the worm's many mandibles can also  provide protection. It is said the lizard's saliva, which covers the  worm's exterior portion, contains several toxins itself to help deter  other creatures.

The lizard's entire  digestive tract is essentially replaced by the worm's body. Fusing  together and becoming one, through which it transfers nutrients to one  and the other. The lizard's respiratory and blood circulatory system  does most of its respective work, which is shared through the same  connection the worm provides its immunity and nutrients.

The  worm's mouth is lined with almost a hundred teeth-like mandibles, which  it can use to slowly pry away small chunks from whatever food is within  its range. Often other animals, as it's primarily carnivorous. The pair  uses its large size, weight and claws to overwhelm and hold down their  victims, but often does not outright kill them, as it proceeds to slowly  devour them bit by bit. It is speculated the lizard's toxic saliva,  covering the worm, contains paralyzing agents which are transmitted  through the worm's bite.

Most of the  worm's soft body sits safely inside the lizard's body. Its slightly  tougher head can extend out from the lizard's mouth to help increase  reach. To not get pulled out of the lizard it is lined with hundreds of  forward facing barbs on the front. This also helps protect the more  sensitive connections further down inside.

Some  rare cases of extreme growth in the parasite has led to severe  disfigurement to the lizard's body and may cause erratic violent  behaviour. Which may ultimately lead to the death of both. The worm can  not sustain itself anymore without the protection and mobility of the  lizard, nor can the lizard without the digestive and immune system of  the worm. Both have evolved to become fully reliant on each other.

Even  their reproductive cycle has fused together. The worm's eggs embed  themself on the lizard's eggs as nodules before they are laid. When the  baby lizards break their shells, so do the nodules break. Covering the  newborn lizards in parasite larvae who find their way inside to continue  the cycle.