Some fanciful stories about kelpies speak of them appearing in equine form wearing riding tack, complete with silver stirrups and bridle bit. These folktales claim that cutting the harness from the kelpie's body grants the bearer power over it, or causes the kelpie to sicken and die. In truth, doing so has no ill effect on a kelpie, suggesting these stories are spread by kelpies themselves to further trick prey into making foolish mistakes.
Recall Knowledge states that you generally get information relating to what the creature is best known for. But what if what the creature is best known for, as seems to be the case with the Kelpie, is wrong?
Do you still only get that misinformation on a critical failure with Recall Knowledge, even though it is misinformation that is deliberately spread around? Or does it change your typical results in some way?
I'd say that it is valid for a critical failure of recall knowledge, similar for saying, say, an Ice Demon being vulnerable to fire.
In my opinion, as this is a secret roll, the characters (and players) should want to believe the information you are telling them, and be willing to act upon it (or not, in some cases). It otherwise defeats the surprise behind the information.