It seems BOUO was still young when the show began. It has grown noticeably since the first season and gone through a few very strange fashion phases. Teenagers… *eyeroll*

Compare these two screencaps - the first from Season 1, the second from Season 2. BOUO is sitting in the same decorative bowl, but it is clearly larger in Season 2.

The story goes that the original BOUO was stolen in the summer break between Seasons 1 & 2 and had to be replaced, but I suspect this was a coverup to prevent any legal action against the producers regarding BOUO-child labor laws. (I'm sure BOUO was always well treated, but you know how it is… once you've been accused of something like that, you're always guilty to some people.)

It has remained the same size since Season 2, so it must now be a fully grown adult BOUO.

BOUO also experimented with various "tattoos". Fortunately, BOUO's hair would grow out, making any markings temporary.

For your consideration, BOUO's grey-side tattoo, early Season 1:

And by mid-season, it was (mostly) gone:

BOUO tried it again in Season 2:

But again, it grew out after a while:

All of these temporary tattoos were probably pretty annoying for the show's producers, trying to keep a consistent look to their small furry supporting character. However, these tattoos were nothing compared to what BOUO did over summer break before Season 2...

A rebellious, college-aged BOUO will do some wild things. It seems BOUO needed a change and decided to try being solid red for a while. Can you imagine the reaction of the producers when BOUO showed up on set like this?!

BOUO convinced them to give it a try and appeared this way for an entire episode (Acceptance). However, some discussion must have gone on behind the scenes, because by the next episode BOUO was back to its original colors.

Looks like BOUO was still living red off-set for at least half of Season 2, though. We can see here that after a long day's shooting, BOUO's "make-up" is starting to crack.

By the end of Season 2 BOUO was comfortable in its own fuzz and no more significant variations in its appearance would be seen. Guess it finally grew up and out of those silly phases.