In Gold and Silver, you could catch one by romping through the Union Cave on Fridays.  
Recapturing one of these named Hewger was the reason that Pyrce attempted to go back in time via a Celebi in Pokemon Special, and it helped return one of Santa Claus's possessions in the Holiday Hi-Jynx episode of the anime.  In Gold and Silver, you could catch one by romping through the Union Cave on Fridays.  Also in the anime, Ash caught one in the same episode that Tracey (+) Sketchit teamed up with him and Misty.  In the Fossil series of the CCG, it had 80 HP and could execute both Confuse Ray and Water Gun. (*)  Its relative rarity is apparently due to being hunted to near-extinction, mainly because it has a "gentle heart" and "rarely fights", while in times of happiness it is reported to "sing an enchanting cry".  In Red and Blue you were given a Level 15 one as a reward after you beat Gary in the Silph Co. building in Saffron City.  FTP, name this Loch Ness-like turtle Pokemon, good for ferrying people across the water.
Recapturing one of these named Hewger was the reason that Pyrce attempted to go back in time via a Celebi in Pokemon Special, and it helped return one of Santa Claus's possessions in the Holiday Hi-Jynx episode of the anime.  
{Lapras}