I have a feeling you're going you have to do a bit of conversion, whatever you get. (I take it you've seen the
FW riders?)
How about barded horses, for something different?
Fireforge do some. Look, they've got half a gas mask on already.
Thing is Fireforge might be a tad smaller in scale, or at least less bulky, than the average
GW mini. There's a comparison with a bretonnian knight at the bottom of the blog post
here, where you can see the size difference. Although bretonnian horses are a bit chunkier anyway.
I remember someone did a fantastic conversion of an inquisitor on a barded bionic horse, years ago. My google-fu is failing me, though. Gimme a while.
Edit:
Jérémie Bonamant. How could I forget that? Thought there were more photos around, though. There might be on an old site archive, but it's in flash, so I'll have to get off my tablet.
Edit edit:
here's the page free of excess jiggery-pokery. I don't know if the first part, about the building of it, ever featured on the site (I missed the first part when it was featured in ye old, defunct Harbinger magazine too) but that should be plenty of photos to look at the thing.