So, how do we keep going with the first ability? Well, here’s an idea: The iconic creature for green is hydra. Make a larger one each turn.

Okay, there aren’t that many hydras with X in the casting cost. Only eight actually.

There’s also [scryfall]Ivy Elemental[/scryfall], [scryfall]Slime Molding[/scryfall], [scryfall]Wildcall[/scryfall], [scryfall]Wurmcalling[/scryfall] and [scryfall]Krakilin[/scryfall]. So, that’s not something we can actually base our strategy on. On the other hand, some of these aren’t even that good, so maybe this is not the way to go. Some of these we definitely do want, but not all. [scryfall]Genesis Hydra[/scryfall] can be especially good with Selvala, since the trigger is a cast trigger and if its a creature, its resolved first, so you might be able to draw two cards in a row.

However, you are playing monogreen. Its not hard to cast bigger and bigger creatures in each point of the curve. There are also ways to help this process out:

To help out the first and the last one, maybe put in a [scryfall]Plaguemaw Beast[/scryfall] to get some proliferation action. Maybe also use some artifact proliferation. They could maybe help bridge the cap with two drops in the deck.

These will also help the hydras, although they might not really need the help.

We would also like to leverage the mana we can get from Selvala’s activated ability somehow, so these might help:

… and just a bunch of creatures with power above the curve:

Plus any number of Eldrazi Titans, if you feel like you need the top end.

Some cards can help keep up the drawing:

You might even often want to pick themselves up and play them again.

Since we want the chain to start early, all the one-mana elves we can find:

That’s nine, which doesn’t quite feel enough, but should suffice.

Finally, since we already have a bunch of mana dumps, we might as well put in some more ways to produce huge quantities of mana and some ways to leverage the big creatures in other ways. And maybe not some other utility stuff.

… and I’ll throw in a [scryfall]Helix Pinnacle[/scryfall] just for shits and giggles.

After all, Selvala + [scryfall]Helix Pinnacle[/scryfall] + [scryfall]Umbral Mantle[/scryfall] + a creature with power of five or more ewuals a win.

After that we need to cut some stuff to make room for 40 lands, but I think I’ll take out the more awkward X-spells to make the mana less awkward. There probably isn’t enough elves for [scryfall]Wren’s Run Packmaster[/scryfall] either, so maybe [scryfall]Wolfbriar Elemental[/scryfall] instead.

The lands we want are probably these:

Which leaves room for 33 Forests. You should probably test a bit whether the manabase holds, because Selvala is pretty unique.

The final list:

[deck title=Selvala’s Menagerie]
[creatures]
Arbor Colossus
Arbor Elf
Bane of Progress
Birds of Paradise
Boreal Druid
Caller of the Pack
Cloudthresher
Deadbridge Goliath
Elvish Mystic
Fyndhorn Elves
Genesis Hydra
Ghoultree
Hooded Hydra
Hunted Troll
Hydra Broodmaster
Hydra Omnivore
Lifeblood Hydra
Llanowar Elves
Magus of the Vineyard
Mistcutter Hydra
Moldgraf Monstrosity
Nylea, God of the Hunt
Omnath, Locus of Mana
Phytotitan
Plaguemaw Beast
Polukranos, World Eater
Primordial Hydra
Protean Hydra
Roaring Primadox
Silklash Spider
Skarrg Goliath
Soul of the Harvest
Stampeding Serow
Stampeding Wildebeests
Surrak, the Hunt Caller
Terastodon
Terra Stomper
Vorinclex, Voice of Hunger
Wolfbriar Elemental
Worldspine Wurm
Yeva, Nature’s Herald
Yisan, the Wanderer Bard
[/creatures]
[spells]
Helix Pinnacle
Hibernation’s End
Lurking Predators
Utopia Sprawl
Wild Growth
Zendikar Resurgent
Chord of Calling
Momentous Fall
Animist’s Awakening
Enshrined Memories
Gelatinous Genesis
Green Sun’s Zenith
Tooth and Nail
Birthing Pod
Contagion Clasp
Contagion Engine
Umbral Mantle
[/spells]
[lands]
Blighted Woodland
33 Forest
Homeward Path
Mage-Ring Network
Myriad Landscape
Nykthos, Shrine to Nyx
Temple of the False God
Thawing Glaciers
[/lands]
[/deck]