What does Captain Stewart identify as Topthorn's greatest asset?

Stamina....

‘My Topthorn is the finest mount in this regiment, or any other,’ said Captain Jamie Stewart. ‘Joey might be faster, and all right I’ll grant he looks as good as any horse I’ve ever seen pulling a milk float, but there’s no one to match my Topthorn for stamina – why he could have gone on for ever and ever. He’s an eight horse-power horse, and that’s a fact.’