The first mainstream collectible glass bulldogs are the famous Westmoreland glass bulldogs the early pieces had no markings and were made no bigger than 2 to 3 inches in height, except for the more specialised larger glass door stop pieces that came with rhinestone eyes, these pieces started to become popular in the 1920s with the introduction on colour and this in turn created other glass figurine makers from all over Europe to start their own pieces but the maker choosing to copy Westmoreland was a company called RUSSO.
So due to stiff competition the Westmoreland company started adding stickers to their pieces and adding a W mark to the pieces hip.
As this collectible become more popular another company sprang up called TIFFIN GLASS and they produced pieces during the depression of the 20s through to the 40s and could be found by looking at where the buckle had been placed on the piece as it was the other side to the Westmoreland pieces.
These days there are many famous glass makers and artists renound in this field and makers like Lalic Bacarrat and Murano are just a few of the best glass artists in the world and even artists such as Neil Harris here in England can now make a great glass piece at affordable costs and even moved on to bulldog glass pendants and tie pins.
As for investment good glass pieces that are signed or custom made can hold their own value if looked after properly and I would suggest any comical pieces are just bought out of love for collecting or for fun.