Yes, we've seen one of these before. It was an admirable job, but lacking in its vehicle mode. This one is a big improvement, as it's a Master Chief transforming into a Warthog.
It is, of course, a custom job, cobbled together with parts from a McFarlane Master Chief and Actionclix Warthog then built atop what looks like a Binaltech Hound. In Chief mode he stands a bit over 5" tall, and comes with some weapons as well.
Interested? One-of-a-kinds don't come cheap, with bidding on the figure already at US$350.
Halo Transformer on eBay: From Warthog to Master Chief [Gizmodo]