If a character or monster that normally has darkvision is polymorphed into an animal that normally does not, does it still have darkvision?
You don't retain darkvision if polymorphed into a creature without darkvision.
The target's game statistics, including mental ability scores, are replaced by the statistics of the chosen beast. It retains its alignment and personality.
A creature's senses (vision) are a part of its game statistics. Therefore, your darkvision would be replaced by the creature's normal vision.