Yes, Lawrie is genuinely hard done-by here in my opinion, and there is an authorial voice ("scrooge like haverings") directing us not to think so. But it all comes back to the unfairness of Ginty just getting a pony and Lawrie having to use up all her savings to get the same; and although Nicola normally puts up with a lot, it is not terribly likeable to buy a pony Lawrie clearly would have agreed to a share in if she had known. I wonder if there is an element of resentment that Lawrie is so much the more confident rider; it is important for Nicola, for once, not to be the one considered incapable of controlling a flighty beast.