Because the Android and Apple markets are both so huge, many big app-makers will create both Android and iPhone versions of their apps... but there are some, like emoji, that were made primarily for the iPhone and you can only find paid or sub-par versions in the Android market.
Still, do some digging, because you may be able to find that app made compatible for Android by a third party (not sold in the Google Play Store but still available somewhere online), or you might look and see if there is a very similar version of what you're looking for in the Android market.
Let me know the app and I'll see if I can help
edit: I think with Android/IOS products, there's a trade-off... IOS might have a slight edge in terms of apps, but Android has the edge when it comes to customization and compatibility with other systems.