The majority of monsters in Gielinor have a weakness. Studying these weaknesses will enable you to kill monsters more quickly, and in more extreme cases, actually survive!
Note that where a monster has multiple entries, this is because there are several versions of that monster. Perhaps some have different weaknesses, or different levels. Perhaps it is the same monster, but with different forms - more commonly seen with boss monsters who change their weaknesses as part of their battle tactics.
Note that there are no monsters beginning with X.