application.js

$(document).ready(function() {
  getNewHorsePage()
    .done(makeHiddenForm);

  $('#new-horse').on('click', function() {
    event.preventDefault();
    toggleForm();
  });

  $('body').on('submit', '#horse-form', function() {
    event.preventDefault();
    submitHorse(this)
      .done(toggleForm)
      .done(addToList)
      .done(resetForm)
      .fail(showErrors)
  });

  $('#horse-list').on('click', 'a', function(){
    event.preventDefault();
    showHorseInfo(this);
  });
});

function getNewHorsePage() {
  return $.ajax({
    url: "/horses/new",
    method: "GET"
  });
}

function submitHorse(form) {
  return $.ajax({
    url: "/horses",
    method: "POST",
    data: $(form).serialize()
  });
};

function makeHiddenForm(html) {
  $('.container').append(html)
  $('#new-horse-form-container').toggle();
}

function addToList(html) {
  $('#horse-list').append(html);
}

function resetForm() {
  $('.errors *').remove();
  $('form').trigger('reset');
}

function showErrors(errors) {
  $('.errors *').remove();
  $('.errors').append(errors.responseText);
}

function showHorseInfo(horseLink) {
  var info = $(horseLink).parent().siblings().toggle();
  $('.horse-info').not(info).hide();
}

function toggleForm() {
  $('#new-horse-form-container').toggle();
  $('#new-horse').toggle();
}

@larsjx