It turns out this Elden Ring boss eats little animals to heal

gamesradar.com:

The Lands Between is chock full of beautiful and bizarre creatures you might not encounter if you stick to the main road. One of the most interesting and uniquely designed optional bosses you can come across in Elden Ring is the Ancestor Spirit, a pitiful animated carcass of a deer-like creature. But don't feel too sorry for this boss, as it turns out it eats defenceless little animals to heal itself. 

This revelation comes from Souls sleuth Zullie the Witch (via The Gamer (opens in new tab)). The area where you battle the Ancestor Spirit is littered with small animal spirits, and while you can't kill them, the Ancestor Spirit can. In the video, which you can see below, Zullie explains that "One of the spirit's attacks appears to drain life from the animals, restoring its HP by killing off smaller spirits."

Healing isn't the only way the Ancestor Spirit utilizes these small creatures; they can also affect how the boss attacks. "During the fight, the Ancestor Spirit will sometimes collapse and warp away, reappearing somewhere else in the arena," explains Zullie. "Where it warps to isn't fixed, but it also isn't exactly random. It targets one of the smaller spirits and moves to its position." Interestingly, the boss changes its moveset based on what animal is selected. For instance, springhares grant a jumping stomp attack, while boars allow the Ancestor Spirit to use a ramming charge ability. 

It's a neat little addition that could easily not be picked up on during the chaos of the battle. However, the animation for warping is frankly rather disturbing; as Zullie explains, "It lifts one of the smaller spirits into the air and then bursts out from it."

Zullie also reveals some content for the Ancestor Spirit fight that