Ash's Quilava

Ash's Quilava evolved from his Cyndaquil shortly before the Lily of the Valley Conference.

Biography
Similar to Ash's Charizard and Infernape, Cyndaquil was forced to deal with an abusive trainer before Ash caught it, although unlike Charizard and Infernape it did not actually belong to that trainer. At first Cyndaquil had difficulty using its Fire type moves but eventually mastered Flamethrower and was capable of using it with such power that Jasmine's Steelix was roasted within its own Sandstorm. Cyndaquil was used frequently in the Johto region and was left at Professor Oaks lab when Ash went to Hoenn.

While training for the Sinnoh League, Ash brought his Cyndaquil with many of his other Pokémon. However, Team Rocket tried to cause problems and Cyndaquil evolved in order to stop them. After its evolution, Quilava learned Eruption and was also revealled to have learned Aerial Ace being Ash's only non Flying type who was capable of using the move.

Trivia

 * Quilava is Ash's only non-flying type Pokémon to learn Aerial Ace.
 * It took the longest of all of Ash's Pokémon to evolve.
 * So far Quilava is the only one of Ash's fire types that does not evolve in its introduction series so far.
 * Quilava's Blaze ability has never been mentioned in the anime but while it was battling Jasmine's Steelix as a Cyndaquil, it used a Flamethrower that was strong enough to turn Steelix Sandstorm into a tornado of fire, implying that it activated Blaze during this battle. Cyndaquil had been hurt badly be Steelix's Iron Tail and Blaze gets activated when the Pokemon using it has taken a large amount of damage. However it should be noted that special abilities did not exist when this episode was first shown.
 * Pryce's Piloswine is another Pokemon that appeared to use its special ability before special abilities were invented because it seemed to use Snow Cloak when Ash battled against him. Coincidentally, Piloswine was battling Cyndaquil when it used the Blizzard attack that seemed to activate Snow Cloak (although in the games, Snow Cloak only works when it is hailing).
 * Quilava had learned eruption in " an old family blend" but strangely it did not use the move when it battled against Nando's armaldo.