We all know by now that Slytherin =/= evil. We also know that there have been plenty of good and evil wizards in all four Houses, and that even a few Muggle-borns end up in Slytherin.

But the impression I got from the books is that blood prejudice is something of a Slytherin phenomenon. Even as far back as the founding of the school, Slytherin was the only one who supported excluding Muggle-borns.

So my question is this: is there ever any evidence in the books of a non-Slytherin wizard holding prejudice against wizards descended from Muggles? Note that I'm not looking for people whose House is unknown, but someone who is explicitly not a Slytherin.

We know Dumbledore himself did not like muggleborns and was prejudice against them for many of the early years of his life, we also know Dumbledore went to Gryffindor during his time at Hogwarts. We also have seen parents of students whose children are not in Slytherin have prejudices against muggleborns specifically against Hermione as well as the Minister of Magic himself.

This may be a bit out of the intended scope of your question, but Grindelwald was blood-prejudiced and technically non-Slytherin. He attended Durmstrang Institute until his expulsion, so he belonged to none of the Hogwarts Houses. I don't know of any canon that specifically addresses his stance on Muggleborn wizards, but he sought to dominate Muggles through magic, viewing them as inferior beings. It follows that he likely held a prejudiced view against Muggleborn wizards.

Again, I realize this only very technically meets the "a non-Slytherin wizard holding prejudice" criteria; sorry if this isn't what you're looking for.

Maybe

A possible example is former Minister for Magic Cornelius Fudge. Fudge definitely is prejudiced in favor of purebloods:

“You are blinded,” said Dumbledore, his voice rising now, the aura of
power around him palpable, his eyes blazing once more, “by the love of
the office you hold, Cornelius! You place too much importance, and you
always have done, on the so-called purity of blood! You fail to
recognize that it matters not what someone is born, but what they grow
to be! Your dementor has just destroyed the last remaining member of a
pure-blood family as old as any — and see what that man chose to make
of his life!

—Harry Potter and the Goblet of Fire

Unfortunately, we don't know which house Fudge was in. Given his general lack of cunning, and no real evidence of ambition besides his desire to hold onto his office, I would be disinclined to say Slytherin. He strikes me more as the sort who values hard work and fairness, i.e. a Hufflepuff.

More generally, though, blood prejudice is embedded in wizarding society. Slytherins may certainly be the most likely to display it, but it would be extraordinary if they were the only ones.

Molly Weasley, a Gryffindor, may have been a bit prejudiced.

When Ron meets Harry for the first time, and Harry asked if his family were all wizards, he says he thinks so but his mother might have a second cousin who’s an accountant. Since he says this in response to a question about if his family are all wizards, this implies that the second cousin has no magic, and isn’t simply a wizard with a passion for accounting.

“Are all your family wizards?’ asked Harry, who found Ron just as interesting as Ron found him.

‘Er – yes, I think so,’ said Ron. ‘I think Mum’s got a second cousin who’s an accountant, but we never talk about him.”
_{^{- Harry Potter and the Philosopher's Stone, Chapter 6 (The Journey from Platform Nine and Three-Quarters)}}

This doesn’t really indicate prejudice on Ron’s part, since it wasn’t his decision not to talk about this relative. However, it could indicate that his mother found having a second cousin who was a Squib or Muggle shameful enough not to talk about. It’s most likely to be his mother who made the rule, since it’s her relative, and Arthur Weasley absolutely loves Muggles - he certainly wouldn’t want to keep quiet about being related to one.

In addition, whoever made the rule would have needed to have some authority over what the family talks about. Ron’s brothers wouldn’t likely have that sort of authority over the family, especially if their mother disagreed with their opinions. It’s possible that the reason they never talk about the accountant second cousin is unrelated to prejudice, but from what Ron says, it certainly seems to be as a result of him not being a wizard.

All of the Weasleys, including both of the Weasley parents were in Gryffindor, so they were explicitly not Slytherin.

“What house are your brothers in?’ asked Harry.

‘Gryffindor,’ said Ron. Gloom seemed to be settling on him again. ‘Mum and Dad were in it, too. I don’t know what they’ll say if I’m not. I don’t suppose Ravenclaw would be too bad, but imagine if they put me in Slytherin.”
_{^{- Harry Potter and the Philosopher's Stone, Chapter 6 (The Journey from Platform Nine and Three-Quarters)}}

Ron in fact thinks it’d disappoint him parents if he’s in any other house than Gryffindor, and sees being in Slytherin as the worst. The Weasleys were not only not Slytherin, but they may have been actively opposed to Slytherin, given Ron’s reaction to the idea of being sorted there.

While it is widely assumed that blood prejudice in the wizarding world is restricted to Slytherin house, it is not. In truth many people in the wizarding world were prejudiced. This belief just mainly sticks to Slytherin because mostly everyone in Slytherin is pure blood and are believers of blood prejudice. This is because when the creators of Hogwarts were choosing who to teach, Salazar Slytherin declared that he would only teach thoose of pure blood.