From that root monstrum, we get two related English words:
We can see how the m-n-st-r root in the original Latin was preserved in the two English descendants, but turned into m-st-r in the Spanish mostrar, losing the middle -n-.
It’s curious how the sense of awe and wonder, of a God-given message, has been lost as monstrum — the divine omen! — turned into merely demonstrations or just showing, mostrar. Sounds like the modern world, in a nutshell.