Defining your problem better would maybe allow us to form a better answer.
Is it leaking? Soapy water to find the leak and repair.
Or "not standing right" as in falls over? I have one used as an entry that I
have small fence post staked in the ground and 2x4's zip-tied to keep it in
position all month. You walk through it to enter the maze, so it takes a beating.
But when I deflat it, inflate it, it keeps an exact position.
knowing about 4 legged inflatables, Id guess the horse isnt standing up right. I use alot of fishing line and tether it to stakes and whatevers nearby.
If its soft and leaking through the nylon Ive run additional tethers I pierced through the meshing.
If it has holes u can do the spray bottle trick, but I always have luck finding the holes by running my hands all over it. If I feel a cool spot I know where to look. Its hot in Fl so any air flowing is easy to find. You can sew the holes or use a bit of tape.
Thank you all..I have looked for leaks and found a few small ones, I taped them up, and the darn horse still won't stand, I guess I am just gonna fishline him to the fence, to give the look of him standing....That darn blowup was expensive