Animal Super Squad

To unlock Corndog, you need to complete the bonus level 'Chumpy Jumpy', which is picked up about halfway through 'Lemongrass Lumberjack' (even though there's no indication of collectibles in that level).

Once unlocked, just beat any 5 levels with Corndog and you're done.

Below are two videos for where to find the collectible that unlocks the bonus level (skip to about 2 mins for the location) and another for how to actually do the bonus level. I take no sort of credit for either video.