This is only a short input for increasing awareness of the topic and reporting experiences.
Our own dog had severe symptoms of nausea after a few hours on a trip from the coast where we climbed in some hours over 2500m. Altitude sickness wasn't the first thing we thought of, but luckily we soon remember it. After a return journey including a 500 meter descent, Cita felt significantly better. Apart from nausea and restlessness, Cita had nothing else and the next day she hadn't had symptoms anymore.
Another case was reported to us: This dog had breathing problems and nosebleeds after a short walk in high altitude. But special: that was after several days over 3000m. After the descent he also felt better, but when he was examined by the vet he was diagnosed with pulmonary edema. He is doing well so far, but needs time and rest so that his circulation can recover.
We don't want to spread panic, but the topic is important to remember. It's hard to find online descriptions about specific symptoms in animals. Pay attention to your animals' discomfort and symptoms that are known from humans while traveling in high altitude. Take care to make slow ascents and enough acclimatization (in humans problems can appear from 2000masl or more).