Sandsplash

Sandsplash was a female Sandslash that Alice bought from the Magikarp salesman at the PokéCenter outside of Mt Moon in Gen 3.5 (Randomized FireRed). She was obtained at level 5.

She had a Lax nature, Flame Body as an Ability, and was last seen holding Brightpowder.

History
Sandslash was acquired because the Mob wanted to see what the Magikarp salesman was selling, which turned out to be Sandslashes with Flame Body and Flamethrower. Initially limited by her level, once Sandslash had caught up to the rest of the party, she became an important member of the team. Although her usefulness began to decline with the loss of Flamethrower and the rise of Slaking, she remained in the party until the end of the game.

Name
“Sandsplash” is a play on the species name Sandslash, combined with the Splash attack that Magikarp are famous for.

"Flameslash" is also a play on the species name Sandslash, but instead combined with the ability Flame Body. The nickname "Flamesplash" is a combination of the above two names. As for "Full Metal Flameslash", it was because at some point in the late game, Sandslash learned the Steel-Type move Metal Claw.

Personality/Lore
Sandsplash was often shown to be wearing a Magikarp Outfit, leading to the interpretation that Alice was scammed into thinking it was a Magikarp, possibly due to her not wearing glasses at that moment.

In spite of that, Sandsplash was depicted as having some Magikarp qualities, like swimming, despite the fact that she was originally a Ground-Type Pokemon and mostly used Fire-Type attacks.

Trivia

 * In regards to the Flameslash nickname, in addition to having Flame Body & Flamethrower as an attack, it was shown throughout the playthrough that Sandslash could have learned Will-O-Wisp via TM, and Ember via Move Tutor, even though neither of these wound up happening.  It was also later discovered that Sandslash's Hidden Power ability was of the Fire type.
 * In Gen 4.5 (Randomized HeartGold), the stream discovered, during the battle against Alice, that Sandslash was given Sacred Fire instead of Tackle, in order to gain power and because she had no Fire-Type moves at the end of Gen 3.5.