Thrinaxodon

Thrinaxodon are confirmed to appear by Tim Haines. They were at one time called "Burrowing Creatures".

Episode 5.5
It is possible the creatures Becker was shooting at in this episode where Thrinaxodon.

Episode 5.6
It is very probable that the Thrinaxodons will appear in this episode, when the team do a lot of time travel. Another possibility is that Becker will be in the present where he was last seen in Episode 5.5 and shoot the creatures he was previously seen shooting at, and they could be, very likely, be either Thrinaxodons, or Arthropleura.