Unicorn companion for the Healer class (from the Miniatures book).

[LordBlade]

So, it says that the companion is a Celestial Unicorn. Does that mean I just take a basic Unicorn from the MM and add the Celestial Template to it?
Or is there a specific entry I need to look up?

[KillianHawkeye]

It's a standard Unicorn with the Celestial template added onto it, yes.

[BowStreetRunner]

It is a Unicorn with the Celestial Creature template plus some extra abilities detailed in the Healer's Companion sidebar on page 10 of MH.