This appears to be some kind of adult slang term, or maybe it's internet slang, whatever the case may be I can't find a definition in any dictionary. Is it an onomatopoeia?
It is an onomatopoeia, not the name for an object unless the author/speaker uses it as such for his own aesthetic purposes but this would be fairly rare.
It describes the way a long object dangles, stretches, lies down, etc. in a lazy manner. The long object coud actually be anything from linguini to a cat stretching its body, from hair to stretching cheese, etc.
If OP saw/heard the word in adult content, it would probably be about a male organ.
ぴろーん is an onomatopoeia to describe a long thin rolled object extends and runs like a tonge of a chameleon. It's nothing to do with adult slang terms.
maybe its a "loooooong" penis?