I'm a mostly indoor cat so I don't have to go on walks to do my business, but if I did, I think this is how I'd do it.

Of course, if we had pasture room for a pony, we'd have to think about all sorts of plants poisonous to our hooved and strictly vegetarian friends, like:

• hemlock
• yellow start thistle
• oleander
• ground ivy
• yew
• castor bean (but not oil)
• rhododendrons
• locoweed
• black locust
• lackspur
• bracken fern
• horsetail
• red maple
• potato and tobacco leaf
• chokecherry
• acorns

I found this University of Illinois database of plants toxic to animals that's worth browsing if you've got green-eaters in your pet family!