In Illustrator...
Group the Unicorn if it's not already a group.
Then in the Appearance Panel add a new stroke to the group. Click and drag the stroke
in the Appearance Panel, so that it is below the <Contents> item in the panel.
You'll then have a stroke around the entire group which you can adjust.
(Old screenshot... but the panel isn't really that different today.)
Photoshop : Layer Style > Stroke will add a stroke around an entire layer or smart object layer. Although, for vectors using Illustrator or Sketch is probably better.