How is he doing now? Is his head at all tilted? Any balance issues? I’m just trying to first of all rule out the possible “bad” causes for one ear to drop, namely an ear infection or e cuniculi. A middle ear infection wouldn’t be visible inside his ear because it’d be inside the tympanic membrane (ear drum). Sometimes you notice cold-like symptoms such as frequent sneezing. If he has balance issues, you should call the vet as soon as they open to book him an appointment, preferably the same day.

As long as he’s eating, drinking, playing and seems happy, this is not a true veterinary emergency. There are “innocent” reasons why one ear can drop.

A bun can hurt his ear or rather the cartilage/tendons around the base of it, making it drop temporarily or permanently. One of my buns got a droopy ear after a fight with my other bun. No wounds or scratches or anything, the ear looked perfect, and it went back up after a few days. Same thing could probably happen if a bun bumps into sth while playing.

Another possibility is that he is part lop and his ears are starting to lop now. Some buns only get one loppy ear, that’s known as “helicopter ears”. It’s extremely cute and of course perfectly normal.

Totally was going to make the same points as bam – well covered ^

Maybe you could post some photos of him? That show the ear ? Would help for us to see how the ear is sitting ( the crown of his head ect ) so we can see if it’s the whole ear lopping / if he appears part lop / if it’s the upper half of ear drooping etc ect
Would help us get a better image of what is happening for your wee bun
In the meantime any change in his behaviour ( decreased intake or decreased output / behavioural changes / head tilting / weakness ) then straight to the vet as bunnies can go downhill fast if it’s illness related

Not saying you shouldn’t eliminate the serious stuff because of course you should but I just wanted to share my experience and point out that it might be nothing to worry about. Bramble had up ears, then all of a sudden one came half way down then all the way down but he could still put them both up – after a while he could put each ear down in turn but never both together then when he was almost two, one day he had both down! He has total full range movement of his ears its so weird, I had him checked out several times and he is totally healthy he just has bizarre ears lol

Some pictures of his different ear positions when he was young so you get an idea. I still to this day cannot get a full lop picture because he is so inquisitive that when the camera comes out his ears go up (or at least one does) because he is super interested in what’s going on

From your photos he does not look Rex … are you sure they showed you his parents ?? I would think they have one parent wrong … to me he looks mini lop cross Flemish !

If he is only a baby he has a lot of growing to do ( can see he is a baby and needs to grow into his ears still and his face hasn’t broadened yet ) I would expect he will grow to between 4lbs and 6lbs ( not a Flemish giant size as he is crossed with a smaller size mini lop so should keep him medium size which those genes )
He’s super cute!
The ear may stay down – other may follow – or it may go up and down
Lops ears can do that too depending on their crown – being crossed with a up earred breed can often effect the cosmetic crown meaning the ears go up and down for sometime before staying down and for some they never end up fully lopped which is cute too

This is just my opinion though – I won’t take him to a vet for a check up as some illnesses can cause droopy ears
I’m going off photos and your description of no other issues or symptoms
A vet would be able to confirm if I’m correct about lop x Flemish or if there is more to be concerned about

Agree with Oakley and Bostons Mama, that’s not a rex at all; the ears are huge and the head shape isn’t what a rex should be, definitely looks like a Flemish, maybe not even crossed with anything..

The other day my girl’s ear went down and she was shaking her head frequently too. The following day she was back to normal. So hopefully this is a similar situation where something was irritating and then eventually it just goes away. Did you notice any head shaking?

Meet Cinders:

http://s36.photobucket.com/user/axg…d.jpg.html

She was adopted by me in November 2013 at around 6 months old (guess by rescue centre). She has always had dead straight ears which would fall back onto her back when she was lying relaxed.

Recently (Last 6 months or so)

They have been doing whatever they want, she’s perfectly health, eats, drinks, poo’s etc normally, i’m not sure on the reason behind it but I think it’s kinda cute 

http://s36.photobucket.com/user/axg…5.jpg.html

http://s36.photobucket.com/user/axg…4.jpg.html

http://s36.photobucket.com/user/axg…w.jpg.html