SR ex-LSWR horse boxes
These horse boxes ran on the SR and naturally, on other companies' systems when horses had to be moved. A key aspect is that the SR never built any horse boxes new but converted its pre-Grouping ones by replacing panelling on the body sides with steel sheeting. It kept them going for a long time.
Peter Tatlow included drawings of them as-built in his "Historic Carriage Drawings" Vol.3 NPCS on pp.88-89, along with pictures of the rebuilds. I have just come across (in the loft) an old copy of Model Railway News, the May 1952 issue which contained an article by R.E. Tustin about LSWR boxes and I have reproduced the illustrations because the drawings show the conversions.

Drawings of the two versions as modernised by the SR with the older, short one below, and the longer one with fodder compartment above. Source: R.E. Tustin, Model Railway News, May 1952/
click on the picture for an enlargement

The long ex-LSWR horse box, No 2821, captured in 1947. Photo: D. Cullum, Model Railway News, May 1952.
click on the picture for an enlargement

The short ex-LSWR horse box, No 2738, also seen in 1947. It's remarkable that an old design like this without a fodder compartment should have lasted this long. Photo: D. Cullum, Model Railway News, May 1952.
click on the picture for an enlargement
In service
I was hoping to provide some train pictures but find myself struggling! It's also an awkward fact that the SR seems to have rebuilt all its inherited horses and made them look very similar indeed.
New addition:

Here, however, is a service picture of one on the SR in an old postcard, at an unknown location and date, with L12 No 424 hauling a two-coach Ordinary Passenger train, with NPCS fore and aft. The loco was fitted with oil-burning equipment in 1926. Photo: Michael Bowerman collection.
click on the picture for an enlargement

An enlargement shows more of the vehicles:
HB |
horse box |
ex-LSWR |
short, as-built |
Fish van |
|
LNER |
1927 design |
------------ |
|||
BC |
tri-composite brake |
ex-LSWR |
|
BT |
3rd brake |
ex-LSWR |
|
------------ |
|||
OCT |
open carriage truck |
|
with a milk tanker? |
BV |
bogie van |
ex-LSWR |
|
When built, these short horse boxes had two windows for the groom's compartment, as seen here. One was plated over when repanelled by the SR

This old postcard by Henry L. Salmon is undated and at an unrecorded location. The loco is an ex-SER Class B1 4-4-0, number hard to see - either 206 or 208? Behind the tender there are two rebuilt short horse boxes which look like ex-SER. Photo: Henry L. Salmon
click on the picture for an enlargement

To show that SR horse boxes travelled on the LNER, N7 No 471 is heading north near New Southgate with three loaded boxes at the head. Most unusually, stock from two companies is running together. The leading box is another short one (ex-SER), the other two, ex-NER, possibly a coincidental working along the axis to and from Newmarket.

Another illustration on the LNER, behind D2 No 3049 as it emerges from Greenwood Tunnel with a quad-art outer suburban train. The horse box looks like an LSWR design but is actually a very similar ex-LBSCR one and it does have a fodder compartment. Photo: C.R.L. Coles.
Click on the image for an enlargement