<parent>
  <div class="line">
    <div class="bullet"></div>
  </div>
</parent>

.line {
  --color: #000;
  --size: 12px;
  position: relative;
  width: 60px;
  height: 2px;
  background: var(--color);
  transition: 0.2s ease;
}

.bullet {
  width: var(--size);
  height: var(--size);
  position: absolute;
  right: 0;
  top: 50%;
  border-right: 1px solid var(--color);
  border-top: 1px solid var(--color);
  transform: translateY(-50%) rotate(45deg);
}

parent {
  width: 100%;
  height: 20px;
  display: flex;
  align-items: center;
}

parent:hover .line {
  width: 400px;
}