The founder-acharya of ISKCON (the Hare Krishna movement) was notorious for denying the moon-landing, but he had other interesting ideas, too. My source is "Prabhupada in his own words: Biology and Nature" by Willem Vandenberg, unfortunately only available if you register at the site Academia. I didn´t know the swamiji was making contributions to cryptozoology, but there you go!
SB 5.23.3, purport
Regarding the great eagles mentioned in this verse, it is understood that there are eagles so big that they can prey on big elephants. They fly so high that they can travel from one planet to another. They start flying in one planet and land in another, and while in flight they lay eggs that hatch into other birds while falling through the air. In Sanskrit such eagles are called syena. Under the present circumstances, of course, we cannot see such huge birds, but at least weknow of eagles that can capture monkeys and then throw them down to kill and eat them. Similarly, it is understood that there are gigantic birds that can carry off elephants, kill them and eat them.
SB 1.8.34, Mayapur, October 14, 1974
There are very, very big, big birds. They are floating in the air, very, very big eagle. They fly from one planet to another. Their resting place is… They start from one planet, and they go and rest in other planet. And they lay their eggs while flying. That eggs also become a bird simply by air cohesion. That’s all.
SB 6.1.45, Laguna Beach, July 26, 1975
There is a big eagle bird. We have no information. They are very big bird. They are flying in the sky, and their rest is from one planet to another. Just like here you find the birds, they are flying from one tree to another. Similarly, there are so big birds… They are called garuda. So garuda, these birds, they start their flying from one planet and sits in another planet. Just try to understand what is their flying. Not only that, they also lay eggs while flying, and the eggs, while falling down, it becomes another bird. And these birds can pick up elephants for eating. So this is God’s creation.