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You can obtain the pet after reaching a specific amount of kills on the relevant boss assuming you have not already obtained it.
On the flip side of this there are a large amount of players who dislike RNG as they feel it's unfair that they may have killed a boss numerous times already and not received a rare drop; Vitalis from Vorago is a good example. This will allow players who have dedicated a lot of time to killing the boss to still be rewarded at the end of it.
Something to bare in mind is that even though this will be a 100% guaranteed drop, the target kills required will be extremely high.