Experts believe the animal, which was seen around the Outer Hebrides, could be a cross between a Risso’s dolphin and a bottlenose.
It’s the latest in a series of suspected hybrid dolphins seen in the area over the past few years.
Potential hybridisation between free-ranging Risso’s dolphins and bottlenose dolphins was first reported by researchers from Whale and Dolphin Conservation off north-east Lewis, where seven sightings of atypical dolphins were photographed in the area between 2010 and 2013.
Further sightings have also been made in more recent years.
The latest sighting was by Peter and Rach Hazlehurst, close to the shore near the Butt of Lewis lighthouse on 15 May.
They were able to take photographs of the unusual animal, which was part of a pod of bottlenose dolphins that also included one adult Risso’s dolphin.
There were three calves in the group, one of which resembled a Risso’s dolphin but was swimming close to an adult that at first glance looked like a bottlenose due to its markings.
However, on closer inspection, the animal was seen to have a shorter beak and more rounded and pronounced head shape than normal bottlenose dolphins – features more typical of a Risso’s.
Mr Hazlehurst said: “There were dolphins swimming around and feeding, and we were there taking photos when we realised we had captured Risso’s dolphins swimming in association with bottlenose dolphins.
“We are still buzzing from this incredible encounter. What a great day we had.”
Both bottlenose and Risso’s dolphins are found around the Outer Hebrides, although Risso’s are more common in the area.
Experts say the latest sighting could shed new light on cross-breeding among dolphins in the wild, which has also been seen in other locations.
“These images help to show the importance of the area for a variety of species of dolphin, and sightings such as these will help us to understand more about the possible hybridisation of the two species,” said Chiara Giulia Bertulli, sightings officer for marine charity Sea Watch Foundation.
“While it is unknown why Risso’s and bottlenose dolphins choose to mate together, one interesting feature is that we seem to have a group of bottlenose dolphins with just one adult Risso’s.
“This suggests that it is a two-way process and there are Risso’s dolphins which occasionally leave their own pods and join up with bottlenose dolphin pods, as well as the other way around.
“Whilst this has been observed from time to time in the past, a clear understanding of reasons for it is not yet available.”