Discover Brazil's Wild Wonderland
This YouTube video shows the annual horse trek occurring in late June as part of the São Pedro de Joselândia celebrations. Joselândia is a small town in the middle of the Barão de Melgaço Pantanal sub-region which, in the wet season is accessible only by boat.
This four day horse trek is a way for the locals to get together - sharing their love of the Pantanal environment, horses and lifestyle. It's also a great opportunity to see wildlife, appreciate the local camp cuisine, and join the fun of the local country dances. This probably isn't for everyone - especially if you're not fluent in Portuguese, but it might be something to try if you want a truly different Pantanal experience. However, you'll need to make arrangements locally including the hire of horses.
Link to further video (in portuguese) at Globo.com
Other Pantanal regions run their own horse treks (Cavalgada) in Portuguese. If making arrangements through a local tour operator, they may be able to book you on one of them - organise a guided day trek specially for you.