Monster

Monster is the official term for an NPC which the player can attack. They are found all over Gielinor in RuneScape Classic. Monsters can be attacked using melee, ranged or magic attacks and are often fought to gain experience, loot, or progress through quests or specific areas. While some monsters will never begin a fight with a player, many high level creatures are aggressive, potentially killing unsuspecting players. Some monsters are human beings, they can be friendly NPCs, and player can sometimes choose methods of interacting them which do not involve combat, such as talking with them.

Combat level
Each monster has a combat level which inform the player of how strong it is. A higher combat level generally indicates that the monster has more Hits and deals higher amounts of damage. It also indicates that potentially the monster has a stronger Defense and will take less damage and be hit by the player less often. It also reflects how strong the monsters attacks will be and how often they will hit the player successfully. Some monsters will retreat from combat when their health is low. If a player has over double the combat level of a monster, the monster will be docile, and will only attack when provoked. In areas such as the Wilderness most monsters are always aggressive.

Players get a color coded indicator for monsters: Note: for calculations the player's actual combat level is not used, but instead a mock combat level is used based on melee stats, hits and a constant of 27, all of which gets divided by 4 for the result. We will call this for simplicity melee combat level.
 * @red@ - if the player is at least 10 melee combat levels lower than the monster
 * @or3@ - if the player is between 7 and 9 melee combat levels lower than the monster
 * @or2@ - if the player is between 4 and 6 melee combat levels lower than the monster
 * @or1@ - if the player is between 1 and 3 melee combat levels lower than the monster
 * @yel@ - if the player has the same melee combat level as the monster
 * @gr1@ - if the player is between 1 and 3 melee combat levels higher than the monster
 * @gr2@ - if the player is between 4 and 6 melee combat levels higher than the monster
 * @gr3@ - if the player is between 7 and 9 melee combat levels higher than the monster
 * @gre@ - if the player is at least 10 melee combat levels higher than the monster

Players who have at least the same melee combat level as the monster will get as the default left click option "Attack" otherwise their default option is "Talk-to", nothing much interesting for the majority of the monsters, specially if they are aggressive.

Trivia
In the early days of Runescape Classic, both options would appear, being the one shown in the left, the fast left click option and the one on the right the alternative option.