The meaning of "heavy-topped and none too sound hocks"
‘By Jove! How well you look!’ he cried, without salutation. ‘I didn’t know you rode.’
‘I used to once,’ she replied. ‘I’m all soft now.’
They swept off together down the ride.
‘Your beast pulls,’ he said.
‘Wa-ant him to. Gi-gives me something to think of. How’ve you been?’ she panted. ‘I wish chemists’ shops hadn’t red lights.’
‘Have you slipped out and bought some, then?’
‘You don’t know Nursey. Eh, but it’s good to be on a horse again! This chap cost me two hundred.’
‘Then you’ve been swindled,’ said Conroy.
‘I know it, but it’s no odds. I must go back to Toots and send him away. He’s neglecting his work for me.’
She swung her heavy-topped animal on his none too sound hocks. ‘’Sentence come, lad?’
This is from "In the Same Boat " by Rudyard Kipling.
I don't understand the meaning below---
--- --She swung her heavy-topped animal on his none too sound hocks.
'heavey-topped' means big head of the horse?
'none too sound hocks' means its hind-legs are hurt?
I am glad if someone would kindly teach me.