Nessian Boar — ruling 1

If a creature can’t block Nessian Boar for any reason (such as being tapped), then it doesn’t block. If there’s a cost associated with having it block, the player isn’t forced to pay that cost, so it doesn’t have to block in that case either. If there’s a cost associated with having it block Nessian Boar, but not to block other attacking creatures, that creature is free to block one of those other creatures or to not block at all.

Nessian Boar — ruling 2

If a creature is required to block two or more different creatures, its controller chooses which one that creature blocks.

Nessian Boar — ruling 3

Nessian Boar’s second ability triggers for each creature blocking it.

Nessian Boar — ruling 4

Nessian Boar’s second ability resolves before combat damage is dealt, and players have priority to cast spells (such as the cards the defending player just drew) after the ability has resolved but before combat damage is dealt.

Nessian Boar : règles, fonctionnement et interactions (MTG)

Nessian Boar is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

All creatures able to block this creature do so. Whenever this creature becomes blocked by a creature, that creature's controller draws a card.

How it works : Nessian Boar

Original English text — Wizards of the Coast / Scryfall

#1January 24, 2020· Wizards of the Coast
If a creature can’t block Nessian Boar for any reason (such as being tapped), then it doesn’t block. If there’s a cost associated with having it block, the player isn’t forced to pay that cost, so it doesn’t have to block in that case either. If there’s a cost associated with having it block Nessian Boar, but not to block other attacking creatures, that creature is free to block one of those other creatures or to not block at all.
No other card is named in this ruling.
#2January 24, 2020· Wizards of the Coast
If a creature is required to block two or more different creatures, its controller chooses which one that creature blocks.
No other card is named in this ruling.
#3January 24, 2020· Wizards of the Coast
Nessian Boar’s second ability triggers for each creature blocking it.
No other card is named in this ruling.
#4January 24, 2020· Wizards of the Coast
Nessian Boar’s second ability resolves before combat damage is dealt, and players have priority to cast spells (such as the cards the defending player just drew) after the ability has resolved but before combat damage is dealt.
No other card is named in this ruling.