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When the sun finally broke through around oneish ,i decided to have a quick look in my local woodland (Orlestone forest ,Kent ) after a tip off ,that a nice abberant Comma had been found along one of the rides ,the day before ....................no joy on that one ! but i did see several
Green veins and small Whites ,2 Brimstone ,2 Commas (standard !),2 Speckled Woods ,1 Small Heath ,1 Red Admiral and 5 Peacocks ,amongst them was this very odd deformed specimen ,which i at first thought had its hind wing missing ,but it is present but very tiny ............on an otherwise perfect insect.
Regards Allan.W.

I'd say the hindwing failed to inflate upon emergence, perhaps a blocked or damaged vein stopped the fluids from being able to be pumped in to it. I had a Brown Hairstreak with similar issues this year.

Yes Bugboy ,you,re theory had also crossed my mind ,and is probably a fair explanation ...............that said the wing does appear to be fairly flat ,and not crumpled up ,as generally seen when the wings haven,t inflated properly ....................still no matter ,a very unusual Peacock! Allan.W.

Seen a few similar to this (but never with such an under-developed wing). I recall finding a Dark Green Fritillary a few years ago with an under-inflated hindwing. Sadly, the upperside was also slightly deformed so this one couldn't fly.