Strands #803 uses the clue "Weaselly wascals" to point at mustelids, the animal family that includes otters, ferrets, martens, polecats, badgers, and wolverines. The spangram is MUSTELIDS, and the remaining theme words are all members of that family.
Source check: the official NYT Strands service confirmed the May 15, 2026 spangram and theme words. URLs checked: https://www.nytimes.com/svc/strands/v2/2026-05-15.json ; https://www.nytimes.com/svc/strands/v2/2026-05-14.json ; https://www.nytimes.com/games/strands ; live intent search for Strands #803 / May 15.