If you mean another trainers Pokemon you have to use cheat codes
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It is a wild Pokemon, so no, it does not belong to a trainer. You can't catch other trainer's Pokemon.
If you are in battle with another trainer you cannot catch that Pokemon that belongs to them. But if you find out where that Pokemon's habitat is (by using the pokedex) then you can go and catch it with a poke ball, great ball, ultra ball etc...
The only way is to get a action replay card and use catch trainer's Pokemon when you fight your rival.
you need a cheat that can catch a trainer's Pokemon to have lapras. because there is a trainer that has a lapras.
you cant because you need to catch it.