This is a skill point.
You need the Morph-O-Ray gun before you can attempt this trophy, which you will not have access to the first time you visit Rilgar though, the gun is obtained at the end of the mission on Planet Oltanis. The tanks are the things that look like tanks and are spraying green gas about the place. This trophy sounds hard but it really isn't, firstly you will want to do this with one of the tanks that is out in the open so you can flank it (or one where you are at a higher area so you can jump on top of it). Then just fire the Morph-O-Ray until the bar on the right fills right up to the top, thus turning the tank into a chicken. Another easy way to get this skill point and trophy is to wait until you get the 02 mask as the tanks gas cannot effect you, therefore you can just stand there and spray it until it turns into a chicken. Trophy and skill point will unlock shortly afterwards.