In my experience, I’ve received them randomly from Portkeys. I’ve also seen them (less frequently) on the ground during some kind of event (Brilliant Event, Community Day, etc.). They still seem to be somewhat rare.
Jenni Russell Answered question
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Erumpent Horns
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I venture to say Niantic has removed Erumpent Horns from the game. I have not heard of anyone finding them in 10 days