I seem to recall reading somewhere that this is actually a really stupid thing to do, and you just risk getting the wound infected instead.
 
Also. one of the properties that mythology assigns to unicorn horns is an ability to neutralize poison. Presumably, this would be amplified in the case of an alicorn, and a bite on the horn would be especially protected via this property. Plus, could a venomous creature even gets its fangs/stinger into the horn in the first place? It’s probably much tougher than skin and flesh.
 
And I have never in my life heard of a poisonous snake that has eyebrows and a mane, nor any confirmed sightings of one that talks. (Talking snakes are generally constrictors.)

Is this how she got those plates?