It wasn't until the release of modal double-faced cards (MDFCs) that we retroactively referred to these as transforming double-faced cards (TDFCs)
If you get enough spells, or multiples of them, just like Wild Mongrel it can be one of the best cards in your deck

Doomsday: Out: 1x Shattering Spree, 1x Damping Sphere, 2x Fable of the Mirror-Breaker In: 2x Grafdiggers Cage, 2x Unlicensed Hearse Bazaar Decks: Out: 3x Maddening Hex, 1x Trinisphere, 1x Shattering Spree, 3x Damping Sphere, 1x Chalice of the Void, 1x Pyroblast In: 2x Alpine Moon, 3x Fury, 2x Grafdiggers Cage, 1x Pithing Needle, 2x Unlicensed Hearse Additionally, if you suspect they have Collector Ouphe: Out: 2x Pyroblast In: 2x Dead // Gone Oath of Druids: Out: 3x Damping Sphere, 1x Shattering Spree, 2x Fable of the Mirror-Breaker, 1x Legion Warboss In: 2x Alpine Moon, 2x Dead // Gone, 2x Grafdiggers Cage, 1x Pithing Needle Creature-Based (Humans, etc.): Out: 3x Pyroblast, 3x Maddening Hex In: 2x Dead 3x Fury, 1x Pithing Needle If you dont think the opponent has good activated effects, like if theyre Mono W Humans without Cathar Commando, you can board in Unlicensed Hearse instead of Pithing Needle

Marvel, Kamala Khan Marvel Super Heroes $0.35 White Widow, Free Agent Marvel Super Heroes $0.59 Pym Particles Marvel Super Heroes $0.35 Night Nurse, Healer of Heroes Marvel Super Heroes $0.79 Shuri, Wakandan Inventor Marvel Super Heroes $0.35 Quake, Agent of S.H.I.E.L.D
Even more impressive, she gives all your creatures with +1/+1 counters trample, making her a perfect leader for decks focused on counter synergies
The 3W Sliver gives all Sliver creatures you control an ETB trigger that pumps their stats +1/+1 for that turn