A sexually explicit work of fanfiction involving My Little Pony characters. In the fandom, several members did ponyspeak and used "clop" as a term for pony masturbation. Some members of the MLP fandom do get sexually aroused by the characters, and even go as far as to masturbate to the erotic fiction. Hence, the word "clopfic" was used.
P.S. Do not assume all MLP fans are like this.
A clopfic is a fan fiction story that contains explicit sexual content, often involving anthropomorphic characters. It is a type of fan fiction that is generally adult-oriented and not suitable for younger audiences.