Ash's Lapras was the first Pokémon that Ash captured in the Orange Islands. Originally it was found washed up on a beach but Ash managed to help nurse it back to health until it was kidnapped by Team Rocket. Ash managed to free it and captured it until they could find Lapras' family. Lapras was primarily used as the gang's transport throughout the Orange Islands, but it was also used in several of the Gym battles and the final League match. Ash eventually let it go with its family, but saw it again when in Johto and helped it defend a lab and the other Lapras from Team Rocket.
Squirtle was the first of Ash's Pokémon to be caught by Ash off-screen, making Lapras's capture the third in the English series and second in the Japanese. Ash only caught his 30 Tauros on-screen in the Japanese series; as this episode was banned in the English series, the Tauros simply appeared.
Ash obtaining Lapras may be a reference to the first opening, where he was seen riding one.
In The Electric Tales of of Pikachu, despite the events of Lapras being similar to that of the anime, Lapras stays with Ash in this version.
Lapras was the first Pokémon that Ash had that got separated from its mother and then returned to her later on. The second is Larvitar.
Coincidentally, Lapras reunited with Ash in the same episode that he received Larvitar's egg.