An adder is a common mildly-venomous snake found in most of Europe.
Morfin Gaunt was known to play with a live adder while singing to it in Parseltongue (HBP10).
"The name 'adder' is derived from nædre, an Old English word that had the generic meaning of serpent in the older forms of many Germanic languages. It was commonly used in the Old English version of the Christian Scriptures for the devil and the serpent in the Book of Genesis" (Wikipedia).
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