The Merman costume is the male equivalent to the better-known Mermaid costume. It is exceedingly rare, with many versions of it seen being tongue-in-cheek renditions of female mermaids but worn by men.
However, merman costumes do exist, and the quality of the outfit can fluctuate wildly. Like with the mermaid costume, one readily-apparent difficulty is having a mermaid's tail that still allows you to walk. This may be an easier costume for someone who's giving out candy than for someone walking around to collect candy. Another issue is that typically a merman costume consists of going shirtless, which can be a problem in colder environments.
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A far less costly Merman costume, though the quality level drops significantly as well.