These are adults. Immatures are duller with a shorter horn. These huge, bizarre birds are related to waterfowl. They get their name from the spiny "horn" sticking out of the top of their head earning them the nickname "unicorn of the bird world." These screamers can be heard making loud braying calls that sounds like a donkey and carry long distances. They are armed with a carpal spur on their wings used in combatCanon PowerShot SX60 HS.
References:
Carboneras, C., Boesman, P. & Kirwan, G.M. (2019). Horned Screamer (Anhima cornuta). In: del Hoyo, J., Elliott, A., Sargatal, J., Christie, D.A. & de Juana, E. (eds.). Handbook of the Birds of the World Alive. Lynx Edicions, Barcelona. (retrieved from https://www.hbw.com/node/52790 on 19 July 2019).
Piland, N. (2010). Horned Screamer (Anhima cornuta), version 1.0. In Neotropical Birds Online (T. S. Schulenberg, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/nb.horscr1.01