I think it was, before tunes were ever defined as Rap Rock' a' la Runs
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Runs meaning RunDMC colllaboration with Aerosmth
I really woulden't say Freddie raps in the song....
Technically in We Will Rock You, it's singing, because the notes Freddie hits can be transferred to a piano (unlike a rap song, which is just talking)
I think Dinah Mo Hum by Frank Zappa or The Revolution Will Not Be Televised by Gil Scott Heron would probably qualify as one of the first, if not the first. Although I'm sure there are many that I'm not aware of...