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)?
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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.