Even better! And like many parasites, it has a complicated lifecycle.
The horse-head snail seeks out a human in which to inject its eggs. Then the implanted eggs affect the human's behavior and body chemistry, making them wistful and prone to wandering, at the same time emitting a pheromone that is irresistible to dragons. The dragon then kidnaps the human and is influenced by the pheromones to guard him/her.
The eggs hatch and another stage of unicorn parasite then infects the dragon. This stage releases eggs into the dragon's digestive system which are then excreted.
When knights come to rescue the hostage, their noble steeds are then infected by the unicorn snail eggs which migrate to the brain, eventually devouring it and growing a shell-horn ovipositor (If the eggs happen to infect a paladin's steed, the resulting brain snail also induces the horse to sprout wings).