Unless otherwise noted, assume that monsters are medium-sized, unarmored, have speed 6 (can replace an attack to move again, or replace either one of those to retry a save), attacks have Reach 1, and roll 1d20 for all saves (though some monsters may have advantaged / disadvantaged saves or checks when appropriate).
Default Monster Stats
GMG v1 p. 24
Related Rules
3Armor VarietyMonsters / Building Monsters
Monster ArmorMonsters / Building Monsters
Monster BuilderMonsters / Building Monsters