I have noticed that my 6 month old Standard Rex is getting whats called I believe an dewlap, Fat underneth her chiny!.. But rest of my rabbit's dont have it.. Why is that??
I have never had a Rex myself but I believe the Standard Rex usually have big ones? All the ones I have saw have so I figure it is fine.
As for show a dewlap is allowed in does. Depending on breed they can be big or not. Like French Lop can have a pretty big one while a Holland should not really have one. I believe a decent size one is allowed in Standard Rex but it can not be so big that it does not look right.
I am not an expert on dewlaps, but I think that some rabbits just get them. It may depend on the breed etc. but i believe it is quite natural.
It can obviously be a sign of being overweight, and in that case, dieting would be the best solution.
Dewlaps can make cleaning that area quite hard, so you need to check around it to make sure it's not icky, especially as she could be accidentally 'dipping' it in her water bowl/muck on the ground.
Having done some brief research on showing guidelines (just now lol), it shouldn't be a problem for your Rex to have a dewlap as long as it is not 'excessive', being the word used.
So that should be fine.
Np. Like I said, I am no expert at all when it comes to showing/breeding etc. But from what I've read, this seems absolutely fine, unless she's looking a bit podgers too