I just a quick search of the available literature, there are a few hundred studies involving creatine monohydrate supplementation in humans, there were 0 (zero) documented cases of anything resembling a 'rash' or a similar allergic reaction.
Which does NOT mean, that you (or the other poster) did not get a rash after taking a white powder out of a can labeled creatine monohydrate
supplements are basically unregulated, you can put whatever you want in a container, label it whatever you want and sell it. legally. Even here in Europe, where we have much stricter regulation than in the US
So whenever you get an allergic reaction to some white powder, stop taking it.
But if you insist o the fact that 'creatine monhydrate' gave you a rash, you have to provide some sort of documentation and/or explain why you dont get a rash from the 120+ grams already in your body but from the few grams that you add by supplementing
what is well documented, however, is manufactures putting whatever they think will get an effect in all kinds of supplements. Some national teams are forbidden from taking any kind of supplement except those provided directly by the team, because people keep failing drug tests because of not-so-clean supplements (there was a recent case with the German team at the last winter olympics, a few years back the entire german under-17 soccer national team failed a drug test, almost 30 people, because a powder to make carbohydrate-mineral-vitamin recovery drink was tainted with a stimulant)
for example, almost all companies (even without any ill intend) use at least some sort of filler (most of the time starch based) and possibly a drying agent (usually some sort of mineral salt), anticaking agent (some salt) and preservative
because supplements are not 'food' they dont fall under the same set of rules and the manufacturer doesn't have to label whats actually in his product.