Riot Games explained the reason behind Nunu's name change to Nunu & Willump, in Riot Games' "Ask Riot: 100th Edition."
The question brought up the fact that Quinn and Valor, Sejuani and Bristle, and Kled and Skaarl remain separate.
Ryan 'Reav3' Mireles explains that Nunu & Willump are two friends working together and neither of them is more important than the other. The other champions use their mounts more like tools instead of a partner in the game.
For a 100th edition, the segment didn't cover many topics or even new information.