Feel free to ignore this if uninterested, this is mostly for referencing for myself anyway
Wikipedia- there are tons of different pages for creatures here though you will have to look on multiple pages and dig around to find the obscure stuff
https://en.wikipedia.org/wiki/List_of_legendary_creatures_by_type
A book of creatures - lots of really obscure crytids with illustrations
https://abookofcreatures.com/
Cryptid wiki- Cryptids count as mythical creatures right?
https://cryptidz.fandom.com/wiki/Cryptid_Wiki
It's something Wiki- it's kind of similar to the crytid wiki
https://itsmth.fandom.com/wiki/It%27s_Something_Wiki
Karkad