The Hellpuppy is a pet dropped by Cerberus.
When a player receives the pet, it will automatically try to appear as their follower. At the same time, a red message in the chatbox will state You have a funny feeling like you're being followed.
However, if a player receives a pet while having a follower out (for example, a cat), it will be placed into their inventory. When this occurs, the message in the chatbox will instead state You feel something weird sneaking into your backpack. If a players inventory is full and they have a follower already, they will not receive the pet.
On death, if a player loses their pet (as a follower or in your inventory) it will wander around for 30 seconds before disappearing completely.
Players can pay a one-time fee of 500,000 coins to Probita in East Ardougne to have this pet insured. Should the pet be lost while insured, it can be reclaimed for 1,000,000 coins.
For an exhaustive list of all known sources for this item, see here