Weresnakes, also known as Wereserpents, are any of a number of therianthrope strains that have the ability to partially or wholly assume the form of a snake.
In Dungeons & Dragons, wereserpents are native to the Forgotten Realms setting; they debuted in Dungeons & Dragons 3rd Edition in the Serpent Kingdoms splatbook, and were then updated to Dungeons & Dragons 4th Edition in the Forgotten Realms Campaign Guide. Sadly, due to its status as a member of the "gimmick" werebeasts, we know very little about them. In 3rd edition, we are told that weresnakes typically are lithe, slender and finely featured in their human form, with dark hair and eyes, as well as a tendency to avoid body piercings or jewelry that is affixed to the body instead of their clothing. No such information on their looks appears in 4th edition. Wereserpents are capable of assuming the form of both a giant snake and a human-snake hybrid, although the precise details of that form are unclear; their 4e lore states that the result is a bipedal reptilian humanoid with slender, flexible limbs and a long tail.
Faerunian wereserpents hide their condition as much as they can, and tend to be most common in areas with a high yuan-ti presence, suggesting that they are the result of the yuan-ti's experimentation into melding humans and snakes. In 3e, it is stated that they are most commonly found in yuan-ti settlements and in cities (especially tropical ports) where yuan-ti slavers and agents operate. In 4e, it's stated outright that both yuan-ti and sarrukh know how to transform humanoids into wereserpents. In 3e, it's stated that wereserpents tend to despise yuan-ti and nagas, but in 4th edition, they are described as common lackeys for powerful reptilian creatrues, such as yuan-ti.
One bit of Realmslore that remains consistent in both 3rd and 4th edition is that there exists a fallen Tethryian noble house, Clan Janor, whose members are all wereserpents, which they have exploited to try and rebuild their power base as merchants despite being scattered up and down the Sword Coast. In general, wereserpents tend to seek powers through mercantile or criminal fields (or both), and they are often rogues or arcane spellcasters.