Apple doesn't allow broad-scope (like console) emulators because they promote software piracy & copyright infringement (because of the game ROMs. Emulators alone are perfectly legal, as they have similar legal stance as VCR's, CD/DVD-burners & other recordable / re-writable mediums). As a result, you'll NEVER OFFICIALLY FIND ONE IN THE ITUNES APP STORE.
In order to run a console emulator on your iOS device, you will either have to Jailbreak your phone (WARNING: VOIDS WARRANTY & greatly increases your risk of bricking your device) OR find an emulation website that's compatible with your iPhone's web browser (Safari is the default option, but Firefox is available as well) to run it from.
The only reason you can find emulators on Android is that Google allows you to access your device's storage space (or use a microSD card) to place whatever you like on it. As long as the Android application itself doesn't do anything illegal (like provides the ROM files for you), Google generally doesn't care. Google also gives users the option to allows "side-loading" (installation of apps from an outside source), but this isn't enabled be default (you have to enable it yourself) as they warn you that it can open up your device to extra security hazards (as Google cannot monitor app installers outside the Google Play Store).