This is a lovely set of studies; I particularly love how you have captured the stretchy, tight look of the bats' folded wings, it looks natural.
The biggest stand out to me, is the pug in comparison to the others, it looks quite flat and lacks volume in comparison to the rest of the studies. Adding value in accordance to the geometrical shapes that make up the dog, sphere for the head, cylinders for legs etc, would help a great deal. I also think the bend of (the pugs right) right paw looks a little too bent also.
All in all though, it's a wonderful set of studies with tons of character; my personal favorite is the bat for sure. The gorillas fur looks great, and the gradual value change really shows volume. The texture and colours of the iguana are solid also.
Again, nice work, and fab studies.