For this topic the criteria are as follows: The instrument is one that rarely, if ever, appears in rock music The instrument is prominent enough in the song to be an integral part of the song The use of the instrument has to add something special to the song that you wouldn't expect when used in a rock song Easily, in my opinion, I think that medal has to go to AC/DC with It's A Long Way To The Top (If You Wanna Rock and Roll) for the bagpipes. To make the bagpipes sound like they always belonged in rock music is something special.
I was thinking Queen would have a lot of unusual instruments, the ukulele springs to mind, but they tend to be in their not rockin' tracks. There's a lot of unusual sounds on tracks like The Fairy Feller's Master Stroke from Queen II, Freddie playing harpsichord and there's castanets in there too.
Oingo Boingo's "Grey Matter" prominently features some custom made "rumbaphones." They appear throughout the song but take center stage around 2:50.
Ween "Ocean Man" - Mandolin (I'll leave the floor open for someone else to post "Losing My Religion")
the 'ondes martenot' on radiohead's 'the national anthem' ... the ondes martenot https://www.youtube.com/watch?v=v0aflcF0-ys
Not sure the sitar fits OP criteria...was common all over in the 60's music...sowhat the heck ..Yardbirds "Heart Full of Soul"