Another person said that he read a guide that said they were different.
Also, does everybody define a legendary as a Pokemon that cannot evolve from another Pokemon or hatch from an egg, and has a BST higher than 579?
A Mythical Pokemon iis a term generally used to describe Event Legendary Pokemon. Mythical Pokemon have to be obtained by special events that are distributed at certain locations. Where as a regular Legendary Pokemon can be obtain through normal gameplay. These Pokemon can not be found through traditional encounter methods.
Mythical Pokemon Include:
Hope I helped.
Source: Knowledge and Deduction