An interesting addition
Snorunt was a very unique pick for Ash's final Hoenn Pokemon. Not only was it an Ice Type, but Ash also caught it a mere 18 episodes(and one badge) before his first League battle.
Snorunt's pranks and mischievous quirks set it apart from the rest of the team. It was pretty weak in battling, at least when it managed to land its Ice Beam.
In the episode "Rhapsody in Drew", Snorunt evolves. As soon as it evolves, it perfects its Ice Beam attack, and the playful icy Pokemon turns into a powerhouse, with a very intimidating aura.
However, it seems to not have lost its playful streak still and freezes Ash right after in fun.
Snorunt's playful energy somehow goes really well with Glalie's intimidating facade. It doesn't seem out of place. With the masterful yet underappreciated writing skills of the Pokemon, writers-it seems even natural.
The idea of a floating, ice-breathing monster, that loves to play pranks, seems endearing.
Glalie is a case of very fast character development. Viewers are just starting to get used to a naughty new character when suddenly, it evolves.
And now, we have a powerhouse that has already accumulated affection from the audience.
The writers treat it really well, as it goes on to knock out at least one Pokemon in every battle it participates in with Ash.
Even against overwhelming type advantages (Clark's Charizard, Morrison's Metang), it performs really well and comes out on top.
Glalie by itself proves that good character development doesn't necessarily mean lots of screen time. With good writing, that requirement can be bypassed.
In retrospect, Glalie was too good for the Pokemon anime- an anime that has constantly rotating characters. The old characters rarely return for a long time.
Glalie was unfortunately not brought back in the Battle Frontier anime. Although it was present there, it didn't participate in the Lily of The Valley Conference either.
Also, Ash seems to have forgotten about it for the most part. Even when Goh and Ash encounter a nest of Glalie in the mountains in the episode "Absol Absolved", Ash doesn't seem to remember or mention his own Glalie. This is a major pet peeve for most nostalgic fans.
While it could have been handled better, Glalie's character development is one of the best-written arcs in anime. It is certainly a great example of how to handle a late-introduced character.