Those cards are Furnace Whelp, Garruk's Packleader, Raise Dead, and Stormfront Pegasus.

• Furnace Whelp is Pioneer-legal via Magic 2015 (M15), and is on Arena via Jumpstart (JMP).
• Garruk's Packleader is Pioneer-legal via Magic 2015 (M15), and is on Arena via Foundations Jumpstart (J25).
• Raise Dead is Pioneer-legal via Welcome Deck 2017 (W17), and is on Arena via the Arena Beginner Set (ANA or ANB).
• Stormfront Pegasus is Pioneer-legal via Welcome Deck 2017 (W17), and is on Arena via Arena Anthology 4 (AA4) under its printing from Magic 2010 (M10).

Interestingly, all four are Pioneer-legal because they were in a non-booster product, with Furnace Whelp and Garruk's Packleader being in the Magic 2015 sample decks, while Raise Dead and Stormfront Pegasus were in the 2017 Welcome Decks. All cards in the Magic 2015 sample decks are legal in any format where the Magic 2015 (M15) main set is legal, and all cards in Welcome Deck 2017 (W17) are legal in any format where Amonkhet (AKH) is legal. I should add that Sphinx of Magosi was in Pioneer Masters (PIO), and its only printings in Pioneer-legal sets are in Welcome Deck 2016 (W16) & Welcome Deck 2017 (W17).

Attached are screenshots showing what happens when you attempt to include those four cards in a Pioneer deck on Arena, as well as a screenshot with the printing in a Pioneer-legal set for each card.