Benjamin Olson
Hero
I think it all depends on the lore of unicorns in your setting. If you are fully committed to accepting the official lore of unicorns then it's really not going to work. If you feel free to tweak it a bit you can (in increasing levels of lore modification):
A) Say the paladin's horse has been blessed by the unicorn god or whatever, and give it a unicorn horn, ornamental or as a replacement for the hoof attacks.
B) Decide that such specially blessed (or whatever) horses turn into unicorns, and that this is where unicorns come from, and then have the steed gradually change to unicorn stats over the course of the next ten levels or so.
C) Ignore the statblock for unicorns. Just stick a horn on a horse and say that in your world that's all that a unicorn is.
A) Say the paladin's horse has been blessed by the unicorn god or whatever, and give it a unicorn horn, ornamental or as a replacement for the hoof attacks.
B) Decide that such specially blessed (or whatever) horses turn into unicorns, and that this is where unicorns come from, and then have the steed gradually change to unicorn stats over the course of the next ten levels or so.
C) Ignore the statblock for unicorns. Just stick a horn on a horse and say that in your world that's all that a unicorn is.