The stem monstr– is fairly rare in English words; the only other basic words containing it are monster and remonstrate.
A monstrum in Latin was a supernatural event, or an omen: a sign to show that something special was happening. In Roman times, omens were often unexplainable things in the natural world: the strange behaviour of birds or animals, a creature born with a deformity, an oddly growing tree, stars moving across the sky, strange lights, or fires that didn’t burn the thing that was on fire. Over the centuries it became used more and more to refer specifically to creatures. Because unknown or unexplainable things usually make people feel uncomfortable or even frightened, the word monster finally developed the meaning it has now in English.
Remonstrate means to protest forcefully. Re usually means again, back (as in repeat, return), but can be used to mean really, completely (if you do something again and again, you must reallymean it!). So remonstrate means to really show exactly what you feel – to protest very clearly and with feeling.
Other words can be created by adding further prefixes and suffixes to the stem monstr- or to the root word demonstrate:
monstrous – like (ous) a monster
monstrosity – the state (ity) of being like (ous/os) a monster
demonstrable – able to be (able) demonstrated
demonstrative – tending to (ive) show something, e.g. demonstrative pronouns; a demonstrative person shows their feelings clearly
undemonstrative – not (un) tending to (ive) show feelings, etc.