There are different groups of monsters: Golems, aberrations, imitators, slimes and plant monsters (there could be more if someone can come up with them). Monsters do not look like animals. Even if a creature looks like a big rat with way too big fangs, this would still be an animal and not a monster while humanoid looking creatures that are not Post-Eophrims are usually Non-Eophrims, so monsters are their own special thing. Not all monsters are aggressive and instantly try to kill people. A lot of them are, yes, but there are also those that are harmless or just want to protect their territory, just like it is the case with animals.
Golems can be made of more than just stone. They could be made of anything that is non-organic - with the exception of wood golems - like vulcanic glass, metals, sand, the non-organic leather you can find in Nomvul's canyons and much more. They have no sexes since they are no regular life forms. New golems usually just start to exist and researchers are not sure how their whole creation process works. You could have a random pile of stones that could become a golem although this never happens inside of settlements and cities but only in the wilderness.
Aberrations have strange body shapes, unusual abilities and look outright bizarre, some even like abominations. An example would be a hunk of meat mass with tentacles that does not look like any other kind of creature.
Imitators are monsters that take on the shape of things like chests, furniture, living armor etc. while still having eyes, mouths and sometimes other body parts they are hiding when waiting for prey. They can not imitate the shape of living creatures like people, animals and other monsters. Imitators, similar to golems, have no sexes and do not reproduce normally. They reproduce by dividing themselves into new imitators if they have enough energy after eating, so it can be tough to keep their numbers in check.
Slimes are sticky creatures that are either fluid or solid. They can have all different kinds of colors and differences. There are the usual slimes slurping on the ground, ones that jump, other ones that have arms and so on. By the way: Gallerts and slimes have nothing in common and are NOT related in any way. Gallerts are jelly-like, not slimey/gooey and there are no jelly-like slimes. Slimes are usually highly resistant to projectiles like arrows, bolts and bullets although this can be turned around with the right kind of ammunition.
Plant monsters are different from regular plants. While regular plants on Horhsal can be strange sometimes they still act mostly like usual plants while plant monsters are sentient and most of them move around and act like other monsters, meaning they are either aggressive, predators or peaceful. Some of them also like to disguise themselves as regular plants to catch unaware prey.