Bosses with a hard mode have a drop chance that is 5 times higher when killed in that mode. For example, the decaying tooth is 1/1000 in hard mode. Bosses with a challenge mode - e.g. bosses in The Heart - have a doubled drop chance in that mode.
Each boss has an associated threshold, which is a number . For example, General Graardor's threshold is 1000. This number relates to the total kills of the boss recorded in the player's Beasts tab in the Adventures interface.
With the exceptions of Reeves and Tess, the thresholds are all 1/5 of the drop chance's denominator multiplied by the number of pets. For example, the Dagannoth Kings have a drop chance of X/2500. One-fifth of 2500 is 500, but there are 3 pets, so the threshold is 3×500, or 1500.
For every recorded kills the player has of the boss, the numerator of the drop chance increases by 1.
This is rounded down, so having 1999 recorded General Graardor kills is treated the same as 1000 recorded General Graardor kills.
For bosses with a hard mode, the two kill counts are added together when checking the number of times the threshold has been met. Normal mode and hard mode kills are weighted equally; i.e. 1 hard mode kill has the same effect on increasing the drop chance as 1 normal mode kill does.
The numerator is capped at 10, so the drop chance will no longer increase once the threshold has been passed 9 times.
For example, a player with 2464 General Graardor kills and 603 hard mode General Graardor kills has 3067 total kills, so their decaying tooth drop chance numerator is increased by 3, giving a drop chance of 4/5000 (1/1250) in normal mode and 4/1000 (1/250) in hard mode.
In a purely mathematical format your chance of receiving a pet on any particular kill is
is your normal mode kill count and is your hard mode kill count
The majority of these pets were added in the boss pets update on 5 January2015. They are all unlocked by obtaining a specific item drop from the relevant boss. A server-wide broadcast occurs when a player receives a drop. When the player uses the item, the message "As you touch the artefact, a familiar face appears... Congratulations! You have unlocked the pet: <pet unlocked>.", consuming the item and unlocking the pet.
Pets can be obtained while LootShare is active; however, they are not distributed via LootShare. Boss pets are always given to the player who would have obtained the drop if LootShare was not active.
Each of these pets can be examined (by any player) to display the number of kills their owner has against the corresponding boss. They can also be used to override Summoning familiars from the boss pets section of the pets interface. If used this way, the overridden familiar will show the relevant killcount upon being examined.
Base drop chance denominator (hard/challenge mode/Telos)
^This is the "per drop" chance, where Vorago gives 5 drops per kill. As such, the "per kill" drop chance (the chance that at least one is dropped in Vorago's 5 drops) is a between 1/1000 and 1/1001 (or between 1/500 and 1/501 in hard mode). If 5 players each obtain one of the drop slots, their individual chances are 1/5000 (1/2500 in hard mode). See below for source.
^Obtainable only after the player has used the maul of omens on Vorago in hard mode on all rotations.
^This is the "per drop" chance, where Vorago gives 5 drops per kill. As such, the "per kill" drop chance (the chance that at least one is dropped in Vorago's 5 drops) is between 1/20 and 1/21. If 5 eligible players each obtain one of the drop slots, their individual chances are 1/100.
^This drop does not use the kill threshold system to improve drop chance.
^ abcdefDrop chance is not affected by the drop increase reward from reputation.
The Queen Black Dragonling and Dagannoth hatchlings were originally not planned to be released with the other pets, due to the Queen Black Dragon being "too big" and the Dagannoth Kings having old animations. Due to player feedback, they were included with the other pets.
The original concept was to allow players to obtain the pets by either getting the item as a drop, or by gambling reaper points for a random pet. The latter was scrapped later on.
For a short while after the update, you would get a notification in your chatbox saying "Your favourite bosses can now be obtained as pets, so what are you waiting for? Get out and catch them all!" This is a reference to the Pokémon franchise.
In the first batch, all examine texts were plays on their larger counterparts' examines.
^Jagex. Mod Porky's Twitter account. 29 January 2015. Mod Porky: "Who wants to know how many boss pets have been unlocked? http://i.imgur.com/iAdN1hG.png" The referenced table contains the transcription of the attached image.
^Jagex. RuneScape's Twitter account. 22 September 2015. RuneScape: "Confirmed. The Kalphite Grublet is the rarest pet. Gratz if you have one! #RSDevQA http://twitch.tv/runescape https://pbs.twimg.com/media/CPhWtRvUkAEuDVl.png" The referenced table contains the transcription of the attached image.
^Jagex. Mod Ramen's Twitter account. 15 March 2016. Mod Ramen: "Here is a little something for all of you curious about how many people own Vitalis, Bombi and Raid pets https://pbs.twimg.com/media/CdlOKyVW0AAV3SD.jpg"