What extinct animal had horns around its head with a longer horn near its nose, ate plants and lived millions of years ago?

Styracosaurus probably fits your description best.

The triceratops.

Several. Your description is too vague. Could be the brontotherium you're thinking of but actually many had nasal 'horns' and rear skull horns.

Centrosaurus or Styracosaurus

countless animals fit that description, some ancient rhinos, ancient antelope, ancient bovine, and a load of ornithischian dinosaurs fit that description. TBH some theropoda like ceratosaurus fit that damn description