 If you liked
Petshop of Horrors
|
 ...then you might like
Mouryou no Hako
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Very similar feel, melancholic and eerie. Neither series is super well done in my opinion, but they both create a similar mood that I really enjoy. They also have similar aesthetics, with dark colors and sparse animation. I would describe neither as horror exactly, but they're both mysterious and somewhat creepy. Also both exploring the nature of being human in some way. I think if you like one of them, you'll like the other one as well.