If you happen to be the type of person who owns miniatures, you probably have a collection of the basics: a bunch of guys with swords, some ogres with clubs, a few zombies, a dragon, and maybe a cool-looking female rogue with two daggers. If you're really into collecting minis, you might have a an entire shelving unit reserved for your vast catalog of lizardmen sorcerers and bugbear fishermen. But can you really ever have every mini for every encounter?
Sometimes the party is facing a large-sized scorpion-like thing made of blackened bone and chitin, with a tail composed entirely of chattering skulls. Well, Mr. "I own a mini for everything", do you have one of those? Oh, you didn't buy the campaign-specific miniature set? Well, too bad! What are your options?
1) Order the exact mini online. Gotta catch 'em all!
This is a fine answer, provided that you A: have a lot of disposable income and B: have room in your miniature menagerie left. Minis are small, yes, but the habit of collecting them all can get quite expensive and take up a lot of space.
2) Use a stand-in. Spiders look sort of like scorpions, and you have a Giant Spider mini.
This will work in a pinch. As previously outlined, though, it's not the best option. When the battle is over, someone at the table will be remembering it as the "big spider fight", and not the "chattering scorpion-horror" fight. Some people are just more visual people than others.
3) Get creative! Grab some paper and draw your own two-dimensional mini.
Personally, this is my favorite thing to do, because even bad art can be evocative, and it will allow the players to fill in the missing pieces with their imagination. Afterall, even if your players laugh at your derpy interpretation of the monster, they will stop laughing once its hungry claws wrap around their squishy, tasty heads. And hey, if you don't want to draw it yourself, odds are that someone else already has.