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 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 player's inventory is full and they have a follower already, they will not receive the pet; it must instead be claimed from Probita in East Ardougne for a reclaim token, which costs 1,000,000 coins.
On death, if a player loses their pet (as a follower or in your inventory) it will wander around the area for 30 seconds before despawning. Even if the player returns in time, the pet cannot be picked up, and can be reclaimed from Probita for a reclaim token.
For an exhaustive list of all known sources for this item, see here