Mike
You could always buy say G1 cows and paint them to look like Black Dexters which are a smaller breed of cattle anyway. They aren't all black either, some are brown.
tried to import the Thingiverse cow into Solidworks as a solid body but the file is too large. I'm not sure if this is 1% or 1000%, but it won't have it.
Actually, Simon, if you provided internal detail you could take on a nom de plume as an artist. Something like Damien Hirst, perhaps, although I appreciate that no-one could possibly have a name like that. You have to start somewhere, though.......
I might go steampunk and add cogs, valves & cylinders. Perhaps one of those early locos with gab motion could provide artistic inspiration. All to scale of course!
A farmer friend of mine once told to be careful about cows, as some weren't around in the UK in the 1950's. Just shows U can never take anything for granted where historical modelling is concerned.
Postman has been so here's some pictures. Foreground is Schleck, background is Britains. Schleck won't fit crossways unless I decapitate at the base of the neck. Britains is not far off the length but in a side view is completely invisible.
These are the ones I previously bought which I think (not being an expert on cows) are much better:
My previous supplier was Pat Maslin who was in the dolls house business and now retired, but think I may have to give her a call and hope she will reveal her source!
this is the Preiser "standing cow" in front of a GER van. I think it could be fitted in your cattle wagon, though you may need to move its tail and heel. When considering 3D printing costs, or even making them yourself, you probably just need the top half that can be seen, not the udder bits.
PS very nice interior detail on that wagon.