Oh, god. His "Barlowe's Guide to Extraterrestrials" is an old favorite. I forgot how much I loved that as a kid. Need to order my own copy now.
I don't remember a tree/critter in it but maybe this book? https://www.amazon.com/Barlowes-Guide-Extraterrestrials-Douglas-Barlowe/dp/0894805002/ref=sr_1_1?crid=31X981UHY4Y0X&dchild=1&keywords=barlowe%27s+guide+to+extra-terrestrials&qid=1604979286&s=books&sprefix=barlowe%2Caps%2C202&am...
I'm sure this isn't it, but the only book I know along those lines is Barlowe's Guide to Extraterrestrials. That link is Amazon. While there, check the "also viewed" list for similar titles.
Please, please, try to employ more imagination than all those attention-seekers who got themselves interviewed by newspapers back in the 1950’s -1970’s during the big UFO “flaps.”
Science Fiction authors have managed to devise some truly weird aliens. Here is a “field guide” to some of those aliens. Google the title and you can see many of the book’s imagery online.
Look up <em>Barlowe's Guide to Extraterrestrials</em>. Wayne Barlowe did a much better job depicting a Pierson's Puppeteer.