Altcoin is alternate coin.
Describe in one word? The word I choose is babies!
They're like bitcoin's babies  
That roam the cryptocurrency world to help startup businesses.
Usually they're self proclaimed as a better substitute to bitcoin, since they look up to their daddy which is bitcoin.
They were like followers who follow the steps of their father.

Really hard to explain altcoin incorporated in a word I chose. But that is what you're looking for, so I hope you have fun reading this.