You can be very inventive with a 1/3 acre, but you couldn't call it a pony paddock or keep a pony and horse on it without putting some fairly extensive plans into action. However, I love a good plan, so here's what I'd do.
Buy the land - they've stopped making it, and it's always a good buy. Then get DIYing. Is it a flat, rectangular paddock?
You could split it in 2 and put a permanent yard with, say, 2 stables, a tack room and an open-front hay store on one half and have the other as a little turn out pen. If you make sure the ground is well maintained it should stand up to some heavy use. I put down hardcore with hole-y rubber mats in my winter paddock, and then let grass grow through it. I just did the area up by the gate where the horses stand in winter, begging to be allowed back in eating hay and where the water trough is, and it has coped really well. That was with 2 horses and a pony attempting to trash it too.
Alternatively, you could split it into 2 and get a little field shelter on skids, and tow that from one half to the next when you move halves so that you can rest one half, and this way would require no planning permission.
You are never going to be able to keep a horse and pony in the conventional sense on that little ground, but it's a hell of a lot more than we used to keep our ponies on when I was little. You'll get lots of people telling you it can't be done, but it can, you just need to be ingenious and put in a bit more work. If you are happy to exercise your horse most days and feed hay when everyone else's is getting grass, the little bit of land will give you somewhere to keep them and a bit of leg-stretch space for them.
Option 3 would be to buy the land, plant some fruit trees and get some chickens, and rent a place at the livery across the road.
But buy the land! My neighbour wanted to buy about a quarter of an acre from us, and a local chartered surveyor said that even used as just a buffer strip from our land it would increase the value of his property by about £35k. (This is not what we offered to sell it at, btw!) You could always convince OH by getting a valuation report done on the value of the house without the land, and one with the land - if the bit of land costs less than the increase in value you are onto a winner whatever you do with it.