Icehorn is also a tamable hunter pet. Please note that because it is also a hunter pet that even though you may not have seen Icehorn before, it doesn't mean that he isn't already cached by NPCScan. If a hunter has this tamed pet out near you, your NPCScan will cache it. If it is cached in NPCScan and this rare is up your alert will not go off. Be sure to check and clear your cache often!
Also, please have some courtesy. If you come across this mob and a hunter is in the process of taming it - leave them be. Hunters, in order to lessen the chances of someone snagging him while you are taming, hit him first so that you tag him. If a passer-by notices that the mob is grey'd out they are probably more likely to continue on their way.
Icehorn is not a difficult mob to tame or kill, his only special ability is something called Romp.
Another fun and interesting thing to note is that he can be tamed with the ice mist buff that you see in the picture above. Check it out, more info here.