Ash's charmander had already had evolved into a Charizard, but he has charmander in Snow Way Out, which is after Ash's charmander evolved.
1. Its release Schedule was messed up because of the Porygon Seizure Mishap
Snow Way Out!, as it is currently, is a special OVA of Pokemon, which aired on October 5, 1998. However, originally, Snow Way Out! was supposed to be Episode 41 (which was set to air on January 6, 1998), but was rescheduled after the incident with Episode 38 (the Electric Soldier Porygon episode which caused seizures all around Japan). Charmander evolves into Charmeleon in Episode 43 (which first aired on May 7, 1998). So in other words, if you take the episodes in the original ordering, it makes perfect sense why Ash still has a Charmander in that episode - because it still hadn't evolved yet. However, because of the Porygon episode, it was rescheduled, and so when it did air, it was chronologically out-of-order
2 .It is an OVA so it isn't even meant to fit in the Story-Plot
