What a strange juxtaposition. The grafitti artist maybe spotted a rat in their peripheral vision and got subconscious inspiration.
We have lots of rodents in Glasgow too. On the underground you can see mice scurrying about on the tracks, and I once spotted a large rat sitting on the edge of a dustbin near Charing Cross (Charing Cross Glasgow, not London). More bizarre was the squashed corpse of an enormous toad on a west end pavement...
Yesterday I was doing outdoor climbing at Auchinstarry, a reclaimed quarry. There was a dead mouse at the foot of the climb we were doing. We were wondering if it was an omen :D! The poor rodent had probably frozen to death while trying to get back to its burrow. I wasn't able to take a photo, as it was so cold my phone suddenly packed in!