JK Rowling escreveu em seu site que o Summoning Charm só funciona em objetos inanimados, explicando que criaturas e humanos só podem ser convocados com a convocação de objetos que estão segurando ou usando.

‘Accio’ only works on inanimate objects. While people or creatures may be indirectly moved by ‘Accio-ing’ objects that they are wearing or holding, this carries all kinds of risks because of the likelihood of injury to the person or beast attached to an object travelling at close to the speed of light.
_{Welcome to my new website! - J.K. Rowling}

Wonderbook: Book of Spells, um jogo PS3 geralmente considerado canônico, contém esta citação que apóia a afirmação de Rowling.

Once you have mastered this charm, you have the ability to Summon any object of which you have need (Providing, of course, that you respect the International Statute of Wizarding Secrecy since no object should be Summoned if it is likely to whizz under the nose of a Muggle). The Summoning Charm is ineffective on most living creatures, and those that can be Summoned are rarely worth Summoning (such as Flobberworms).
_{Book of Spells (ellipses replaced with spaces), see source video}

No entanto, uma história contada no início do jogo aparentemente contradiz isso, afirmando que Gideon Flatworthy, um homem que era aparentemente bastante habilidoso no Charme de Invocação, conseguiu convocar as vacas que o esmagaram até a morte.

Experts still debate what precisely led to his final, foolish act. Most agree that he had probably become delirious through lack of food although some think that he was determined to die as he had lived. All we know is that, on the eighteenth of September, 1743 Flatworthy attempted to Summon himself an entire farm complete with livestock, cosy cottage and well-stocked larder. Naturally, the buildings would not shift, but the furious farmer followed his flying cows to the cave on the hill and discovered Flatworthy, still lying on his cushions, but crushed to death beneath a pile of hay bales and cattle.
_{Book of Spells (ellipses replaced with spaces), see source video}

Embora seja possível que a história tenha sido exagerada ou fictícia, ela é apresentada de fato e aparentemente foi objeto de opiniões divergentes entre especialistas. Também é possível que as vacas sejam um dos poucos animais que podem ser convocados (embora isso pareça improvável, pois não são mágicos e não particularmente incomuns) ou as vacas em questão estavam vestindo roupas na época. No entanto, este não é o único exemplo do Feitiço de Invocação usado em objetos animados, tanto em histórias prováveis ​​de ficção quanto na realidade. Exemplos de animais sendo diretamente convocado magicamente, em ordem cronológica:

Inside one house the little girl's warts vanished as she slept; the lost donkey was Summoned from a distant briar patch and set down softly in its stable; the sick baby was doused in dittany and woke, well and rosy.
_{The Tales of Beedle The Bard - page 10 - The Wizard and the Hopping Pot}

When the poor little Snidget flew my way I did a Summoning Charm. You know how good my Summoning Charms are, Pru – of course it was easier for me to aim properly, not being mounted on a broomstick at the time. The little bird came zooming into my hand. I stuffed it down the front of my robes and ran like fury. Well, they caught me, but not before I’d got out of the crowds and released the Snidget.
_{Quiddich Through The Ages, pages 12 & 13 (in the writing of Madam Modesty Rabnott)}

NEWT: Accio Niffler!
The case bursts open and a Niffler jumps out.
_{Fantastic Beasts: The Crimes of Grindelwald - The Original Screenplay - scene 48 (reformatted slightly)}

NEWT (of the Niffler): Where is he? Ah, Accio Niffler.
The Niffler is carried by the spell back into the case. NEWT takes the case and dashes off.
_{Fantastic Beasts: The Crimes of Grindelwald - The Original Screenplay - scene 48 (reformatted slightly)}

Harry pointed his wand at the bullfrog that had been hopping hopefully towards the other side of the table – 'Accio!' – and it zoomed gloomily back into his hand.
_{Harry Potter and the Order of the Phoenix - chapter 18}

"Reckon there's salmon, or is it too early in the season? Accio Salmon!"
There were several distinct splashes and then the slapping sounds of fish against flesh. Somebody grunted appreciatively.
_{Harry Potter and the Deathly Hallows - chapter 15}

Existem alguns outros casos em que animais são convocados indiretamente ou sem sucesso, como Newt convocando o Niffler convocando as jóias em sua bolsa, ou Harry não convocando Hagrid e Hedwig durante a Batalha dos Sete Oleiros (que eu acho que podemos colocar até a inexperiência de Harry com o feitiço, mesmo quando ele convocou seu Firebolt, ele estava praticando pouco antes de consegui-lo, e o feitiço fica mais difícil com o alcance). Eu também daria uma menção honorária a Ron por "Accio Brain!".

Mas o que dá? Existe alguma explicação possível para a inconsistência na capacidade desse encanto de convocar animais?