I couldn't find anything about renting a pony. My guess is also, that no responsible person who acutally has a pony would rent it out to have a bunch of kiddos riding in the front yard. Especially not for 30 Euros. That money doesn't even cover the gas prices to transport the pony nor the haggle of putting the pony in the wagon etc. I personally wouldn't do it...
If the theme is gonna be ponys and unicorns why not print out color picture for the kiddos, let them dress up as a pony or a unicorn and have like a little perfomance just like in the circus. Pretty sure they'll have a blast.
Or get them My little Pony toys, that way everybody can have their own little pony and the can play with them after eating and coloring the picture.
Bake cookies in horsey shape... that'd be cool, too. And if you decide to do that, I found out that it look really cool if you put the color straight into the cookie dought and mix it just a little. Gives the dough all kinds of crazy colors. (yeah yeah for everyone who already knew that... I just found out two weeks ago

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hope this helps...