I have had a few ideas for animal costumes that can be made with some waste materials:
I think the best material you can use for a bat is a plastic bin liner, this material is already the right colour for a bat and has the same leathery look about it.
You could cut the bin bag into a wing like shapes and attach them to your back and your wrists to make effective bat wings. You could even use a bag for your body if you want to have a complete look. To make the ears cut triangles in card and paint these black, then stick them to a hairband to make the ears stand up.
Spider or Octopus
To make the legs of a spider or an octopus you will need to use three pairs of old tights. Stuff the tights with anything you can find such as rags or newspaper and then sew them to the trousers that you are wearing. The three pairs of tights and your own legs will make up the 8 legs of the spider or the octopus.
If you chose to be a spider black tights will be best, but for an octopus you could use any colour, just make sure it matches the colour of the rest of your outfit.
To be an elephant you will need a cereal box and toilet roll tubes. Paint all of the waste you have grey, and then start fixing the tubes together, if you make a small slit in each one you should be able to slide them into each other. Keep going until you have the right length you want for the elephants trunk.
With the cereal boxes you will make the ears. Cut a large wiggly semi circle out of the card and tape these onto the sides of a hair band.
If you want a full elephant mask, use some more grey card over your face, making sure you cut out holes for eyes, and stick the trunk onto this.