Colossus

There are countless Beast species, both living and assumed extinct, documented by the Wayfarer Guild. In most cases we even have actual proof of their existence in the form of physical remnants, or reliable stories written and confirmed by the author's peers.
We have no such evidence for the species known as Colossus, which is said to be humongous Beasts wandering some Beast Lands. There are few documented eye-witness statements, all of which are of such a nature that they lack credibility.
Quite frankly, even if we compensate for exaggeration, the descriptions seem unrealistic. As such, it is widely assumed that the stories are pure myths. However, the way these myths are widely spread means that perhaps there is truth to them. If there is, who knows if we'll ever find out.

The Walking Hills

There are many myths talking about walking hills. They speak of entire hills, or even a mountain, wandering up on a country and devouring a leader that was too arrogant for their own good. Some myths even talk about entire countries being consumed. It should be noted here that in the past entire countries have indeed been destroyed by Beasts, but in almost all cases the likely cause was identified.
Some stories talk of drunks that fell asleep on a hill and woke up hundreds of kilometres away, as their hill was actually a Colossus. Others talk about digging wells and accidentally injuring one of these. It is widely assumed these stories are meant as warnings for dangerous behaviour, teaching people how to avoid danger.

Reality

Strangely enough, the Orks too seem to know stories about the Colossus. Honestly this is one of the biggest indicators that maybe they actually exist, as the rest of Orks their precautionary tales all match with actual Beasts known to us. But they too merely know of them and don't seem to know where they actually are. According to engravings we found, we estimate the actual thing would be between 40 and 60 metres long. Perhaps there is value to further investigation, as such a Beast would be a great threat if ever encountered in rage.