A hellcat is a special variant of a normal cat that can be accessed after starting the Freeing Evil Dave sub-quest of Recipe for Disaster. During the quest, players must visit Evil Dave in his mother's basement. There they must use their cat to kill Hell-Rats, which have stolen his mother's special coloured spices which are needed to make the spicy stew necessary to free him. After your cat catches a few rats, it will become corrupted by the demonic nature of the rats and will suddenly transform into a hellcat. As only one hellcat can be owned at a time, any existing hellcats may need to be placed in a menagerie, or transformed back in to an ordinary cat, in order for subsequent cats to be transformed. A hellcat can be turned back into a normal cat of the same colour it was before the conversion at any time by feeding it a bucket of milk.
A cat at any stage of growth can be a hellcat, and a cat that has become a hellcat will continue to age as normal. While it is impossible to convert an overgrown cat into an overgrown hellcat, since overgrown cats cannot catch rats, players can get an overgrown hellcat by allowing a hell-kitten or adult hellcat to age into an overgrown hellcat. Players who have completed Ratcatchers can also get wily and lazy hellcats, either by using their wily or lazy cats to catch Hell-Rats, or by taking an overgrown hellcat to Felkrash and having her train it.
If you are leading a hellcat in Varrock and a stray dog begins to follow you, your hellcat will hiss at it and the dog will flee for a short time.
If you attempt to speak to another person's hell-kitten, it tells you: "Hiss! Go away before I scratch those curious eyes out!"
If you ask your (lazy) hellcat "How old are you now?" He responds "Too old to waste time talking to you about the price of fish." Your character will reply "Fish?," as a reference to how players used to buy items from other players before the Grand Exchange existed.
When asking the cat what it would like to do, the hellcat will proceed to tell a joke with the punchline "Demons are a ghoul's best friend." This is most likely a reference to the common phrase "Diamonds are a girl's best friend," although the player doesn't seem to get this as being a joke. The other thing that the hellcat wants to do is to trip people going off the stairs.
Hellcats appear to be followers of Zaros. If you accept the shoo away option the hellcat says "Thank Zaros for that! Finally, some freedom from your ugly face."