Sounds like the posleen series by John Ringo
http://en.wikipedia.org/wiki/Legacy_of_the_Aldenata
The Posleen, (The aliens) ,look like horses and the main hero in the story uses powered armour and commands one of the armoured units.
Cant remember "Bun Bun" ,(The Anti landing ship mobile gun ,the names a reference to a psychotic mini lop rabbit character from the web comic Sluggy Freelance!) ,being able to disguise itself with foam but its been awhile since I read the series.
All in all a really great read and what got me reading John Ringo's books.