For now, I'm just gonna provide links to Adoptable directories and multi-species pages. I may list out other individual pages later. I may also post sample images of each species if I see interest.
Note: You will now need to be logged into neopets.com to access any petpages.
Adoptable Directories

neopets.com/~Toyest
Toyest Adoptables Directory

neopets.com/~Shecci
The Adoptables Phonebook

neopets.com/~Purmars
Dessert Pets Directory -- Lists Dessert-themed Adoptables
Multi-Species
Adoptables by Shade, neopets.com/~ParasiticPlague

XOX adoptables
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Please feel free to contribute pages! Also, if this top level post needs updating and I forget to check back on this forum, I will likely be reachable on neo as espeons_darkside. Thanks!
More Useful Pages I Maintain (on Neopets)
Neopets Current Events + Resources Page - neopets.com/~events
Tear's BD training guide - neopets.com/~Tear