Helper function for encounters such as UncommonBreedEncounter which call for a random species including event encounters.
If the mon is from the event encounter list, it will do an extra shiny roll.
Parameters
level: number
the level of the mon, which differs between MEs
isBoss: boolean = false
whether the mon should be a Boss
rerollHidden: boolean = false
whether the mon should get an extra roll for Hidden Ability
Helper function for encounters such as UncommonBreedEncounter which call for a random species including event encounters. If the mon is from the event encounter list, it will do an extra shiny roll.