From Eight Tails article:
The lower left part of its horns was permanently sliced off in a battle with A, and the right horns were destroyed by its own point-blank Tailed Beast Ball when fighting the Ten-Tails.
Instead of hind-legs, its lower half consists of its eight tails, which resemble the cephalopod arms of an octopus. These tails will grow back in the event they are sliced off.
If the tails are able to grow back, then why can't the horns too? Since Gyūki is made of chakra, how was it possible to permanently slice off a part of its horns?