The Mule was a "mentalic" mutant of enough power that he managed to overcome the forces of history and disrupt Hari Seldon's plans.
Were such mutants known prior to the Mule? (and known to Seldon)?
The Mule was a "mentalic" mutant of enough power that he managed to overcome the forces of history and disrupt Hari Seldon's plans.
Were such mutants known prior to the Mule? (and known to Seldon)?
The Mule was the first known human to display such remarkable mental abilities in the Foundation universe. He was preceded, however, by at least two - and possibly more - robots; R. Giskard Reventlov and R. Daneel Olivaw. The latter is probably the most important being in the entire Saga, with Seldon unarguably being the most important human. Seldon's robot wife, Dors Venabili, also possesses at least some of the same powers.
As Richard states in his comment above, the Mule was originally from Gaia, where such mental powers were common; it was directly stated in Foundation's Edge that these mental powers were taught to the humans of Gaia by robots, and it is implied in Prelude to Foundation that it is Daneel himself who founded Gaia. So the Mule isn't unique. Even the Second Foundation seemed to possess a weaker version of the Mule's power.
The Mule may have been more powerful than his predecessors; this is never stated, and he is eventually defeated by a weaker practitioner of his emotional control. Given his accomplishments, however, it seems likely that he was stronger than both his predecessors and his contemporaries, though Daneel and Gaia were hampered, even moreso than the Second Foundation, by the necessity of keeping their actions secret from the galaxy at large.
In short: there are no other mentalic mutants known on the Foundation universe.
The Mule was a mutant, in a plot where they are really really scarce (this is not the Marvel universe). And moreover he was a mutant coming from a society (Gaia) that actively uses mental abilities. His are, anyway, unique. Gaians and robot telepathists concentrate on thoughts and communication, while the Mule had a super capability over emotions.
He did not instill the tought that he must be obeyed. He did instill loyalty as an emotion. He can not evem modify thoughts, only emotions, as can be seen on his encounters with Han Pritcher. Pritcher continues being an almost independent thinker, but just ultimately loyal.
In Forward the Foundation several human characters with mentalic powers similar in kind but not extent to the Mule are introduced. The first is Wanda Seldon, Hari's granddaughter, whom Hari discovers has powers when she is 14. Per this summary:
When Wanda Seldon was 14, she asked Yugo Amaryl about how psychohistory was going, and Yugo showed her the Prime Radiant, a device that displayed all the psychohistoric equations. Wanda pointed to a certain set of equations and said it did not look right. Yugo examined it and found it was indeed wrong. Yugo told Hari Seldon about the incident, and Hari Seldon deduced that Wanda had somehow read Yugo's mind and sensed his uneasiness about those equations. At that moment, Hari Seldon conceived of a Second Foundation whose power would lie in its telepathic power and clandestine invisibility.
Wanda Seldon continued to develop her "mentalic" abilities during her life. When Seldon was 76, Raych had gone to Santanni and he needed a new bodyguard, so he hired someone named Stettin Palver. When Seldon was put on trial for attacking someone "without provocation" (according to a lying witness), Wanda used her powers to persuade the witness to tell the truth. What she did not find out for another few years was that Palver was also using his mind there. When they happened to meet, they discovered that Palver was "mentalic" as well, and Seldon sent them to find others with those abilities. They would eventually form the Second Foundation.
Thus we see that at least two people with the ability to read minds and influence people's behavior existed several centuries before the Mule and were known to Hari Seldon.