WikiRhymer has about 2,000 rhyming words and phrases for "mountains" starting with...

fountains
    mountain's
      Pure Rhyme Set #1 of 1 (Add to/Edit Set Members)
      Words that have a pure rhyme on their last syllable only.
      cantons
        plantains
          Atlantans
            End Rhyme Set #1 of 224 (Add to/Edit Set Members)
            cannon's
              cannons
                canon's
                  canons
                    Atlantans
                      Buchanan's
                        Montanans
                          Shannon's
                            End Rhyme Set #2 of 224 (Add to/Edit Set Members)
                            gowans
                              Cowans
                                McGowan's
                                  Palauans
                                    End Rhyme Set #3 of 224 (Add to/Edit Set Members)

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                                    Words that "almost" rhyme on the vowel-based rhyme sound of the stressed syllable like: be/eat or maybe/shapely. (Click for more info)
                                    gowans
                                      Cowans
                                        McGowan's
                                          Palauans
                                            Near Rhyme Set #1 of 2 (Add to/Edit Set Members)
                                            thousands
                                              plowman's
                                                houseman's
                                                  Near Rhyme Set #2 of 2 * (Add to/Edit Set Members)
                                                  Rhymes made up of more than one word. For instance, "jealous" and "tell us" or "shaky" and "make me."

                                                  We have no mosaic rhymes for "mountains".

                                                  Mosaic Rhyme Set #1 of 1 (Add to/Edit Set Members)