Servant of the Scale

Servant of the Scale enters with a +1/+1 counter on it.
When Servant of the Scale dies, put X +1/+1 counters on target creature you control, where X is the number of +1/+1 counters on Servant of the Scale.

Spined Tyrranax

At the beginning of combat on your turn, you may pay {2}{G}. When you do, put a +1/+1 counter on target creature. That creature gains trample until end of turn. (It can deal excess combat damage to the player or planeswalker it's attacking.)

Biogenic Upgrade

Distribute three +1/+1 counters among one, two, or three target creatures, then double the number of +1/+1 counters on each of those creatures.

Jiang Yanggu, Wildcrafter

Each creature you control with a +1/+1 counter on it has "{T}: Add one mana of any color."
−1: Put a +1/+1 counter on target creature.

Thriving Grove

Thriving Grove enters tapped. As it enters, choose a color other than green.
{T}: Add {G} or one mana of the chosen color.

Snakeskin Veil

Put a +1/+1 counter on target creature you control. It gains hexproof until end of turn. (It can't be the target of spells or abilities your opponents control.)

Duskshell Crawler

When Duskshell Crawler enters, put a +1/+1 counter on target creature.
Each creature you control with a +1/+1 counter on it has trample. (It can deal excess combat damage to the player or planeswalker it's attacking.)

Greenwood Sentinel

Vigilance (Attacking doesn't cause this creature to tap.)

Deepwood Denizen

Vigilance (Attacking doesn't cause this creature to tap.)
{5}{G}, {T}: Draw a card. This ability costs {1} less to activate for each +1/+1 counter on creatures you control.

Forest

Ainok Artillerist

Ainok Artillerist has reach as long as it has a +1/+1 counter on it. (It can block creatures with flying.)

Aggressive Instinct

Target creature you control deals damage equal to its power to target creature you don't control.

Mowu, Loyal Companion

Vigilance, trample
If one or more +1/+1 counters would be put on Mowu, Loyal Companion, that many plus one +1/+1 counters are put on it instead.

Forgotten Ancient

Whenever a player casts a spell, you may put a +1/+1 counter on Forgotten Ancient.
At the beginning of your upkeep, you may move any number of +1/+1 counters from Forgotten Ancient onto other creatures.