Unknown brown monster that was never seen

I've encountered a mysterious brown mob in Minecraft version 1.21.51 on two separate occasions.

The first time, I was in a boat with a friend. I looked to my left and saw the mob. It was on a small island almost completely surrounded by water. I watched it move for a short time, and then it vanished behind a tree. There was nowhere nearby where it could have hidden.

The second time, I saw the mob's legs through a small hole in the ground that I was using as a shelter. My friend also heard the sound of it walking on sand nearby. When I went to check outside, the mob had disappeared. The area was flat, so there were no obvious hiding spots.

The mob had brown legs and walked on sand. It vanished very quickly in both encounters. It did not attack me, even when it was very close. It was not a husk, and it was not a visual glitch. It also did not behave like any normal Minecraft mob.

I hope you can help me identify this mysterious mob or explain what might have happened. Thank

you!