A monster type may be referenced by a spell or ability in OSR+, such that the spell or ability has special effects on the type of monster in question.
Humanoid monsters are generally bipedal and intelligent. Most are capable of language and may have culture and live in societies of their own. This includes creatures like lizardmen, fey, orcs, goblins, centaurs, trolls, and even giants.
Wild monsters are sentient, but not intelligent. These creatures are generally incapable of moral reasoning, and so cannot be said to have good or evil intent. This includes both sentient flora and fauna like wild bears and crocodiles, non-sentient oozes and plants, and megafauna like dinosaurs, giant spiders, and direwolves.
Cryptids are supernatural or legendary creatures of varying intelligence. Unlike Wild monsters, Cryptids are either magical in nature, possess extraordinary abilities, or one of a kind. Some cryptids operate in societies; others exist only in lairs or secret or forbidden dwellings. Creatures like dragons, the Sphinx, cockatrices, krakens, and Lamia are all types of cryptids.
Constructs are created beings that may or may not be sentient. All constructs are artificial in nature, whether animated by magic or science. Usually such beings are immune to effects meant for the living (such as poisons or mind-influencing abilities). A Clockwerk sentinel, a golem, or an animated sword are all types of Constructs.
Astral creatures originate from otherworldly realms within the Astral Veil. Such beings include monstrous extradimensional phase spiders, shadows, and elementals made of fire or ice as well as superintelligent celestials and demons.
The Undead are creatures that were once alive and are now reanimated by supernatural means. Such creatures are generally imbued with negative energy that sustains their unlife. They tend to be immune to effects that affect the living, and possess supernatural abilities or extraordinary resilience due to their unliving state. Ectoplasms such as ghosts, humanoids brought back to life because of curses like vampires and werewolves, or creatures animated by dark magic such as zombies are all considered Undead.
Eldritch monsters originate from alien realms beyond the Astral Veil, and typically possess (or are imbued of) psionic power. Such beings are often amoral or totally alien in their thinking, and their physiology is like nothing in the material realm. Cthuloids, brainbugs, overseers, and limbo leeches are all examples of eldritch monsters.