I3 is an enemy in Kirby 64: The Crystal Shards on the N64. It appears as a blocky green boulder with a large eye on the front. They try to drop down on Kirby from above when he approaches them, similar to the Thwomps in the Mario series. Though invincible, they slowly rise back in place after dropping, allowing the player to run past.
The 3 in I3 is believed to stand for cubed since it’s a roughly cube shaped enemy, whilst the I is likely a pun on its one big eye!
Info from Kirby Wiki
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Picture by Link101