They are rare monsters primarily obtainable as in-game secrets hidden on specific map tiles
Having made this list of traits to be found in good four drops I feel even more awkward about a number of cards left off this list that epitomise many of those things
Disclaimer: Recycling policies vary significantly by region and facility
Specifically, the Dragonscale fetchlands, some cards that manage to combine being mega-staples across Constructed, Cube, and Casual formats, low drop rate, thematic importance, and gorgeous art
I feel like they design one prize attackers to need draw backs as if they would be broken without them but then print Lugia + Archeops so, shrug
Some of these creature types continue to receive tons of tribal support, even today