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In the book “All Hell Let Loose” by Max Hastings, the writer mentions about the condition of British farmers during WWII:

"Wiltshire farmer Arthur Street ploughed up his grassland as the government ordered, and sent away his beloved hunter to be trained for harness work. Many riding horses took badly to this humble duty, but Street’s ‘Jorrocks’ ‘trotted home like a gentleman.”

What does "Street's Jorrocks" mean?

T.E.D. is right here. It is mentioned that Jorrocks "trotted home". A "trot" is a type of horse gait. Therefore, Jorrocks is most likely a horse, and presumably Street was the owner of Jorrocks.

Jorrocks is actually a somewhat famous horse name (the name of a famous racing horse), so it makes sense as a horse name (though the original would have been long dead by WW2). I also note the extra apostrophe after "Jorrocks". It could be a typo.

This got to be a bit long for comments, so I'm moving the content here.

farmers sent away their hunters to be trained for harness work. Many
riding horses took badly to this humble duty, but Street’s Jorrocks
trotted home like a gentleman.

This is presumably simply a real-world example of a farmer and his horse, of the type he's talking about. In this case the farmer's surname is "Street", and the horse's name is "Jorrocks".

Most likely the author just included the names to personalize the account somewhat. Make it relatable for people who find generalized historical discussion boring. Some historical writers like to do that. You particularly see this from professional Journalists like Mr. Hastings, who are presumably used to this style of writing in their day jobs. I'm not one who needs or likes this writing approach in historical works, but I have seen it.

Given what Wikipedia has to say about the book's writing style, you should probably expect rather a lot of this:

All Hell Let Loose covers the entire span of World War II, following
the military developments of the war but focusing on the reactions and
experiences of different individuals (both uniformed and civilian).
Reviews refer to the book as an "everyman's story" made up of accounts
from those with lesser roles in the conflict; "ranging from ship's
cooks to wireless operators, farmers and housewives to typists and
black marketeers."

The book addresses several "triumphalist" aspects of written war
history by focusing on the "misery, heroism and endurance" of
individual accounts

(emphasis mine)

I haven't read this particular book, so I can only say that I hope he pulls this off better than the authors I have read who did this. In bad cases it reads like something a history-hating editor forced the author to go back and tack in everywhere.

'Street' in this case is the English farmer, writer and broadcaster A. G. Street, 'Jorrocks' was his horse, and the anecdotes are taken from Street's book From Dusk Until Dawn, published in 1945.

This online edition of Hastings' book has a different version of that paragraph:

"Wiltshire farmer Arthur Street ploughed up his grassland as the government ordered, and sent away his beloved hunter to be trained for harness work. Many riding horses took badly to this humble duty, but Street’s ‘Jorrocks’ ‘trotted home like a gentleman’ ..."

Which is much more clear.

It also includes Street's book in the bibliography:

Street, A.G.

From Dusk Until Dawn

Blandford 1945

Google Books doesn't appear to offer a preview from the English version of Hastings' book, however the Italian version has essentially the same wording as the online version cited above.

I suspect that what has happened in this case is that the edition you have was the victim of some careless editing at some point in its history. Sadly, this kind of thing is not an uncommon occurrence!