@Blue Couldn't you just homebrew a CR 1/2 version of a unicorn and call it a "young unicorn" that can be conjured with the 2nd level find steed spell?

And then if your party ever hits 13th level when the paladin gets 4th level spell slots, you could make a CR 2 version of that unicorn and call it an "adolescent unicorn" that can be conjured with find greater familiar?