As its on the hardstanding, I think this may be the recently arrived stallion' Bingo' from Paignton. He's now got to prove himself with Marwell's mares ( he didn't at Paignton).
Its called a dewlap. All Hartmann's zebras have one- males and females both. You can see it vaguely in this photo but it doesn't identify if its him or not.
The bare patch on the rump could indicate a 'travel sore' from the travelling crate but I'm still not sure as, like you, I'd still expect him to look pink-coloured so maybe its one of the mares.