From ErfWiki

Proposed Canon

Brontoswords are Mount Units used by Goodminton. A brontosword is described as "great lizardly monster", its size as "enormous" and its gait as "flumphing".

Goodminton's Chief Warlord Tommy Firebaugh uses a brontosword as his mount.

Speculation

Brontoswords take at least 2 turns to pop.

Brontoswords may be Heavy Units.

Real World References

Its name seems to be a portmanteau of "brontosaurus" and "sword".

Its way of moving - described as "flumphing" - may refer to a monster from Dungeons and Dragons universe.