While I don't think I can convince you they're real, I'll try to explain why they're plausible designs for a Pokemon game.
Every Pokemon out there is based on something, whether an animal, an inanimate object, a mythological creature, a humanoid sentient, and more.
But Game Freak never makes Pokemon that are 100% faithful to the animals they are based on. This is natural given that Pokemon are cartoons and the designers take a lot of creative freedom in designing them. You could say then that Pokemon are "loosely based" on animals.
Turtonator, for example, is based on a mata mata turtle. But if you compare the two, there are many differences: the coloring, the ability to spew, etc. Game Freak takes a concept or idea, in this case the mata mata turtle, and gives it a cartoony and creative spin. They take it a step further by adding on more ideas and concepts, such as the landmine idea for Turtonator.
So while the designs for the final evolutions of gen 7 starters may seem jarring and "un-Pokemon", they're actually good representatives of what Game Freak has been doing for many years.
Rowlet3 is an owl with a Robin Hood theme. Robin Hood is associated with Sherwood Forest and is known for wearing a green tunic, so the grass type fits a Robin Hood design nicely.
Litten3 is apparently based on Tiger Mask, a beloved heel wrestler persona from Japan. In the reference sheets, we see Litten3 execute several wrestling moves. In traditional heel fashion, he even smashes a table on Pikachu.
As for Popplio3, I suspect Game Freak wanted to make a mermaid Pokemon that wasn't humanoid. For this, they would have to pick an animal. Sea lions are a logical choice. Some people even call sea lions "mermaid dogs", because of how closely their bottom halves resemble mermaid tails. Because of the sea lion's natural physiology, Game Freak can design it with enough creative freedom to make it look like a mermaid while still retaining its basic, fundamental sea lion features.
