"over the preceding weeks we’d seen Niko beat himself over the head with his broom and set fire to his own feet...."
-- Ivan Popa's testimony in the inquiry following the disaster (Pm)
In spite of warning signs, the team manager allowed Nenad to play in the tournament. When his team were losing in the final, he purposely hit a Bludger into a jinxed forest, causing the trees to come alive and stampede the stadium in what became known as the “Attack of the Killer Forest“. Nenad himself was killed by “a particularly violent spruce” (Pm).
Niko is a variant of Nikos (from the Greek Nikolaos, meaning "victory of the people") (Wikipedia).
Nenad (Ненад) is a popular Slavic name (although usually appears as a first name), deriving from the word nenadan meaning "unexpected" (Wikipedia).
Lexicon list of Beaters
From the Web
Writing by J.K. Rowling on Pottermore:
Pottermore enhanced reading experience: Quidditch World Cup
Screenshots of the original Daily Prophet coverage of the 2014 Quidditch World Cup on Pottermore: Imgur archive
Harry Potter Wiki: Niko Nenad