I had some success just using a pencil to separate legs that are too close together. But this is less permanent than some other methods. I don't like doing drastic stuff, especially on aging plastic that may be going through processes. Heat is known to speed up plasticiser issues, and those that have already gone through that process probably won't be flexible enough to change anyway.
Some ponies have uneven hooves. As a kid I used to use a white pony shoe on Windy to make her stand, as one of her back legs is slightly shorter. Though again, not a permanent fix.