Recently I have found (possibly even on this forum) a reference to the following web page
http://www.geocities.jp/f9305710/henkanji.html
on which a number of weird sino-japanese characters are collected.

Unfortunately, the explanations in Japanese are a bit hard for me to follow.
Could someone explain me briefly what this stuff is all about ?

Do those "monsters" exist in the unicode list ?

It's just a list of very rare kanji. Most of which are not in unicode. The red kanji indicate which Kanji dictionary has an entry for the character.

Just scrolled through but it seems most of the characters are just ancient variations of more modern characters that never became widely used but made their way into a dictionary somehow.

Awesome. I learned a 76 stroke kanji that means "To throw up in front of a large crowd". Made my day.

they are not kanji...

I seriously doubt that many of the characters on that page are real at all, in any language, or ever were. They look completely made up.

The ones marked with red kanji at the bottom of the explanations are real enough to be in dictionaries.

Many of them are in the Morohashi or the newer Chinese dictionary; when you look some of them up they're rather suspicious and the only citation is from other (classical) dictionaries. They're not any more "made up" than any other characters but their history of use is slim to non-existent.

Thanks for your explanations. So those characters are mostly from Chinese dictionaries and, even there, their existence is suspicious…
Better forget about them altogether !