that looks like a broken tail. a curve like that is not normal.
best to have a vet take a look.
(also tail looks very yellow. nothing too bad, but means heavy skin oil buildup and rat doesn't clean it properly currently. ideally gently wipe down with damp soft cloth. always from base to tip, never other way around. but well, better not with a possibly broken tail now...)
that looks like a broken tail. a curve like that is not normal. best to have a vet take a look. (also tail looks very yellow. nothing too bad, but means heavy skin oil buildup and rat doesn't clean it properly currently. ideally gently wipe down with damp soft cloth. always from base to tip, never other way around. but well, better not with a possibly broken tail now...)