No, you can catch Entei without catching Moltres.
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Zapdos, Articuno, Moltres, Mewtwo, Entei/Raikou/Suicune (depends on what starter you chose).
four without cheats Articuno Zapdos Moltres Mewtwo plus one of the legendary dogs; entei, suicune or raikou
The only way to obtain them is by trading from Pokemon fire red/leaf green being as you can't trade between 1st/2nd to 3rd generation.
Charizard, Entei, Moltres, Rapidash, and Arcanine are good examples
you have to migrate them from fire red or leafgreen or u trade them form gold silver or crystal to ruby and Sapphire then migrate to platinum