 If you liked
Narutaru: Mukuro Naru Hoshi Tama Taru Ko
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 ...then you might like
Mahou Shoujo Madoka★Magica
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Both series take a genre (pet monsters for Shadow Star Narutaru, magical girls for Mahou Shoujo Madoka★Magica) and deconstruct it by removing every safety feature associated with it. Do NOT make the mistake of showing these to children unless scarring fragile young minds is your intent.
Violence, death, torture, friends killed in front of the main character these are not kids shows despite how the openings look. Both shows are about young girls and there friends and start off as innocent stories but as the episodes go on the shows become darker.
Both seem to be an innocent children's show at first, then turn out to be very dark and disturbing and deep.
Genre deconstruction of their respective genres - for Shadow Star Narutaru, it's 90's 'mons series along like the lines of Pokemon and Digimon, whereas Madoka Magica uses magical girl anime. If you enjoy one or the other for the dark and deconstructive aspects in them, try out the other!