A dog.
A dog of mixed breed; a mongrel.
A bulge, an enlarged part
There's a pooch in the plastic where it got too hot.
A distended or swelled condition.
Her left sleeve has more pooch at the shoulder than the right.
To distend, to swell or extend beyond normal limits; usually used with out.
Inflate that tire too much and the tube may pooch out of the cut in the sidewall.