once you have defeated all eight trainers and are on your way to the indigo plateau you will hit a cave, this cave is named victory road,(or so i believe) while in that cave you will come across a frog looking Pokemon in a spot, there are numerous guides i am sure to find the Pokemon once in there, one of my favourite guide/walkthroughs from Pokemon red/blue are "kodys kick ass Pokemon red/blue walkthrough" this is a good walkthrough, so go you YouTube,
look up
"Kodys kick ass Pokemon red/blue walkthrough finding moltres" its a good guide and i trust it, hope this was helpful!
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If you miss catching any legendary you can't get one unless you have another version trade you the ones you didn't catch Example: Since you missed Moltres then have Blue or Yellow give you a Moltres.
No, you can catch Entei without catching Moltres.
Unfortunately, they have to be traded form Red, Blue, or Yellow (or, perhaps, Green).
Moltres, zapados, and articuno are the three main legendaries in leaf green/fire red
Moltres is in Mt. Mortar