One of the interesting things reading the letters is putting together the jigsaw pieces.  In the summer of 1915 Richard says

Please tell Jane I’ve absolutely cleaned the bath now with Zoy & Brookses & I also washed Susan today in Lux

Zoy & Brookses is presumably a cleaning product, and Lux is still soap. Who or what would you wash in soap?  A bicycle or a dog, perhaps?

On the 30th June, Susan turns up again when Paul writes about a motor accident

I heard from Bee Dudman this morning – she seemed to tell me a good deal about it & that “Susan” is dead – was she in the car & killed or what?

So she was presumably a dog and not a bicycle.

By November 1915, Paul comments to his mother

Susan’s done well! What are you going to do with them all?

So not dead in the motor accident.  This sounds like a litter of puppies; rather a large litter of puppies.

And finally towards the end of November Paul says:

As a matter of fact our Captain wanted a dog – so I told him about Susan’s puppies – but he’s just told me he thinks he’d like an older animal – it’s rather hard to bring them up properly on board – so he’s getting another one.

Piecing together these incidentals are a lot easier with the internet because people, places and brands tend to be mentioned online. But not dogs.