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."
                                                  • around + guns
                                                  • brown + ones
                                                  • brown + sons
                                                  • down + guns
                                                  • down + ones
                                                  • down + runs
                                                  • down + tons
                                                  • town + ones
                                                  • town + runs
                                                  Mosaic Rhyme Set #1 of 1 (Add to/Edit Set Members)