Jackal Familiar — ruling 1

“Can’t attack alone” means Jackal Familiar can’t be declared as an attacker during the declare attackers step unless at least one other creature is also declared as an attacker at that time (either by you or your Two-Headed Giant teammate).

Jackal Familiar — ruling 2

“Can’t block alone” has a similar meaning. Note that the other blocker(s) doesn’t have to block the same attacker as Jackal Familiar.

Jackal Familiar — ruling 3

Once Jackal Familiar has been legally declared as an attacking or blocking creature, how many other creatures it’s attacking or blocking with no longer matters. For example, if Jackal Familiar and Canyon Minotaur both attack, then Canyon Minotaur leaves the battlefield, Jackal Familiar continues to attack even though it’s now attacking alone.

Jackal Familiar — ruling 4

An effect may put Jackal Familiar onto the battlefield attacking even if no other creatures are attacking. That’s because it wasn’t declared as an attacker in this case. Similarly, an effect may put Jackal Familiar onto the battlefield blocking even if no other creatures are blocking.

Jackal Familiar : règles, fonctionnement et interactions (MTG)

Jackal Familiar 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

This creature can't attack or block alone.

How it works : Jackal Familiar

Original English text — Wizards of the Coast / Scryfall

#1October 1, 2009· Wizards of the Coast
“Can’t attack alone” means Jackal Familiar can’t be declared as an attacker during the declare attackers step unless at least one other creature is also declared as an attacker at that time (either by you or your Two-Headed Giant teammate).
No other card is named in this ruling.
#2October 1, 2009· Wizards of the Coast
“Can’t block alone” has a similar meaning. Note that the other blocker(s) doesn’t have to block the same attacker as Jackal Familiar.
No other card is named in this ruling.
#3October 1, 2009· Wizards of the Coast
Once Jackal Familiar has been legally declared as an attacking or blocking creature, how many other creatures it’s attacking or blocking with no longer matters. For example, if Jackal Familiar and Canyon Minotaur both attack, then Canyon Minotaur leaves the battlefield, Jackal Familiar continues to attack even though it’s now attacking alone.
No other card is named in this ruling.
#4October 1, 2009· Wizards of the Coast
An effect may put Jackal Familiar onto the battlefield attacking even if no other creatures are attacking. That’s because it wasn’t declared as an attacker in this case. Similarly, an effect may put Jackal Familiar onto the battlefield blocking even if no other creatures are blocking.
No other card is named in this ruling.