I heard that Missingno had different forms other than the barcode-like version that I've encountered before. Is this true, and if it is, do they have different characteristics other than appearance?
Missingno is complicated since it is a glitch Pokemon. It has 5 distinct forms, but 2 common ones.
The 2 most common are the top left, and the bottom right. Those are usually called the Red/Blue normal form and Yellow normal form respectively for the games they are found in.
Because of the odd nature of the glitch, Missingno actually varies not only between forms, but within forms as well. Certain forms will have different battle cries, or stats depending on a number of different factors (i.e. how it is encountered), and between the Red/Blue and Yellow forms, it will also have a different type (Normal/Bird and Normal/Glitched respectively). The non-normal forms vary at an even wider level in terms of stats and instead copy those values from a party Pokemon.
The Bulbapedia article linked has a lot more extensive data on the glitch, how it happens, and how to get each type and form (and how they all vary), but the general gist is that with the glitch comes some (expected) oddities and inconsistencies.