practically: you'll end up with something like the Ripley/xenomorph clone experiments from Alien: Resurrection
The embryo probably wont even make it to the 4th week. It will die into monstrous tumor thing.
The thing is, there is no single "DNA for horns" it is alot of complicated factor that affect even a single genotype. Something that are "visible" usually the most complex. Like it is a series of different genes in the different part of DNA along in which cells it reside, which one is activated or not, et cetera.
We know alot of the function of a singular gene, like this set produce this protein, et cetera, but we pretty much have no idea how multiple set of genes affect a certain trait of a being. Some overlaps, some are isolated.
Don't even counting the non coding part of DNA because afaik nobody know what they are even for.
So yeah it is not easy.
As for recombinant, we have come so far with simple living beings like bacterias inserted with human genes to produce human insulins or possibly prevent a genetic disease that is caused by a single gene mutation (like sickle blood cell) but anything more complex than that, we barely scratched the surface of the knowledge.
I just wanted to make a joke about a hideous horse/narwhal hybrid begging "kill me" >_>