This metal Space Marine Riding Lizard was from the Rogue Trader era of Warhammer 40K, and is definitely something which stayed in Rogue Trader… Space Marines don’t get about on reptiles in 2nd. Edition and beyond!
White Dwarf #107 answers the question about who sculpted it – in John Blanche’s Illuminations that month he is showcasing David Gallagher’s work and it mentions that he sculpted the dinosaur mount for the RTB01 Space Marines.