"The Horror of Slumber Party Mountain" is not the first horror-themed episode of 'Tiny Toon Adventures'. The show even did the theme earlier on in Season 3 with "Toons from the Crypt" and quite well if not consistently. That doesn't matter though, as 'Tiny Toon Adventures' re-visited the music, sports and film and television homaging themes quite a few times (which classic Looney Tunes did in scenarios that played to its strengths) and generally excelled.
While not one of the best 'Tiny Toon Adventures' episodes by all means, "The Horror of Slumber Party Mountain" homages horror very well and between it and "Toons from the Crypt" this is the better episode. Despite the premise not being as inspired, it's more consistent (helped by that it is a single story episode), has a little more atmosphere, is a little wackier and is closer in spirit to classic Looney Tunes, it also doesn't become too cruel in the way the final quarter of "Toons from the Crypt" did. My view of course.
It takes a little too much time to get going, with it finding its groove once Buster starts telling the story.
Also felt that the identity of the monster while surprising was a bit hard to believe, seeing as normally the person doesn't always have enough personality as a non-villainous character.
However, "The Horror of Slumber Party Mountain" has a huge amount to like. The animation has vibrant colour but also an ominous atmosphere. Especially the freaky character animation on One Eyed Jack and the insides of the mansion. The music is characterful and full of infectious energy and dynamic orchestration, with a haunting edge at times too. The theme song is one of early 90s animation's catchiest.
Furthermore, the writing has enough wackiness and sharp wit and the story has energy and creepiness. The homaging of horror is affectionate and creepy, while never being traumatising. The characterisation is spot on as is the voice acting from some of the most best and most prolific voice actors in the business at the time and ever.
Concluding, very good. 8/10.