(There was a reason why the little horses were riding stick horses, but it only made sense if you were six.)
I made tails by making large, simple tassles & pinning them onto party guests' pants. These were less successful; several just fell apart. But they lasted through the party, at least.
I don't know what traditions are elsewhere, but around here when your child has a birthday party, they receive presents from the guests and hand out "goodie bags" full of edible treats and junky toys of dubious amusement value. I don't like to pass them out and I don't like to have my kids bring them home -- the contents are usually junky but the cost adds up surprisingly quickly. So whenever possible I like to make something that our party guests can take home & play with later. The horse hats seemed just right this year.
Charlotte's birthday is only a week before Halloween, so as soon as I finished the horse hats I moved right on to make George's mummy costume:
(This is his favorite lurching undead pose.)
Here it is with the headpiece:
It's made of strips of muslin, sewn and/or hot glued to a pair of pajamas. (My sister made a costume like this on a sweatsuit base, but she lives in a colder part of the country.) The headpiece is built on a balaclava with the nose bit cut out.
It was a hugely successful costume, and very durable, too -- George spent a lot of the evening hiding under one of the cars, then "rising from the dead" as trick or treaters passed by, and the costume showed hardly any wear & tear at all.
Given the amount of sewing her party had required, I was kind of pleased that Charlotte decided the PERFECT costume was a $6 Target witch outfit: