look at the roping dummies that the cowboys use- they run on a rail- (not the drag behind ones)
you could build something not as heavy or robust- use angle iron for rails.- built like a long ladder.
can be in shorter pieces for ease of transport.
on the one angle iron track, weld on an extra flat bar in height, with gear teeth cut out of it- like an electric gate works- doesnt have to be fancy- cut with a zip disc and a grinder, then a windshield wiper motor from the auto wreckers, and an inboard battery, driving a gear that meshes with the linear gear( the flat bar gears)- thats how it propels itself forward.
now if you really want to be smart- not me!!! uh uhhh - but have the deer on a center pole ( bearings)so that it can swivel( or build a two headed deer like the old "push me pull you" fable) a wiper at each end so that as it passes a point it swivels the deer target, another wiper on the track which activates a trigger to reverse polarity, and change direction on the motor, so it will run back and forth all day.
to be really fancy you can build the legs with joints, attached to cams - so the legs can Run- like the kids toy of the clown riding a bicyle.
sounds like a fun project, cheffie boy