From the Super Mario Wiki
A Unira is an item in Super Smash Bros. Brawl, coming from the game Clu Clu Land. It is a type of Sea Urchin encountered in the said game, acting as an enemy. In fact, it has been referred to as a Sea Urchin in other English localizations, but is known under its Japanese name in Super Smash Bros. Brawl. In said game, the Unira can be picked up and tossed at opposing players. After being thrown, its spikes pop out damaging all except the one who threw it. An interesting note about the Unira is that when someone stands close enough to it, it actually draws them toward itself.
A Unira can change "possession", so to speak, if it is attacked. For example, if Kirby throws it, the Unira will not harm Kirby; however, if Mario attacks the Unira, its properties will change and cause harm to Kirby while not damaging Mario.
 Trophy Information from Super Smash Bros. Brawl
 Names in Other Languages