Culapanthaka (Little wayman) seems to be a likely candidate for someone with ADHD who became an Arahant… 
EA20.12
Then the Exalted One took Cūḍapanthaka by the hand, returned with him to the vihāra and had him sitting down on the spot and holding a bamboo broom, saying to him, “What do you call this object? Pronounce the word for it.”Now Cūḍapanthaka managed to pronounce “bamboo”, but he could not remember the word “broom”, and while he managed to pronounce “broom”, he forgot the word “bamboo”.