Cabin fever

When I was delayed for nine hours (!) at a Tenerife airport after a summer holiday, I saw a lady boarding a plane bound for Madrid holding a carry case containing her dog. It was toy breed-sized, but I’m not sure of the actual breed. I’ve never seen a dog in the cabin on a flight in the UK. Are the rules preventing this? This lady was going from Spain to Spain, so perhaps Spanish internal flights have different rules. I’d be interested to know.
Thanks,

Kate Conway, by email