No animal has naturally "sweet" blood in the way that, for example, sugar is sweet. The idea of "sweet blood" is a misconception stemming from a few different sources:
Misunderstandings and Misinterpretations
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Hippopotamus "Blood Sweat": Hippopotamuses secrete a reddish oily fluid from their skin, sometimes called "blood sweat." This fluid isn't actually blood, and it doesn't have a sweet taste. [Reference: HIPPOS spend most of their day resting in water and can hold their breath for up to 30 minutes. Hippos secrete a reddish oily fluid sometimes called "blood sweat" from special glands in their skin.03-Jul-2017]
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Mosquitoes and Blood Preference: While mosquitoes feed on blood for protein, the notion that they prefer "sweet blood" is inaccurate. Their blood source preferences are complex and vary greatly. [Reference: Monday's medical myth: mosquitos prefer sweet blood http://theconversation.com/mondays-medical-myth-mosquitos-prefer-sweet-blood-10833 (Snippet: She bites to get a hit of protein required for egg development, but preferences for that blood can vary greatly. Some prefer to bite mammals.)] [Reference: Mosquito myths busted – DW – 08/20/2021 https://www.dw.com/en/mosquito-myths-busted-from-sweet-blood-to-schnapps/a-58897280 (Snippet: And he specializes in "blood sucking Articulata animals, basically insects and ticks. ... "That idea about the sweet blood is not quite right," ...)]
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Culinary Uses of Blood: In some cuisines, animal blood is used in recipes, sometimes with added sweeteners to create dishes like sanguinaccio (a crepe made with pig's blood). However, this doesn't mean the animal's blood is inherently sweet. [Reference: Blood & Chocolate | Emiko Davies https://www.emikodavies.com/blood-chocolate-sanguinaccio/ (Snippet: they serve sanguinaccio, which is a crepe, make with the pigs blood and you can have it savory or sweet. ... animals ground up with sugar and ...)] [Reference: Why Chefs Are Cooking with Blood - Bon Appétit | Bon Appétit https://www.bonappetit.com/entertaining-style/trends-news/article/restaurants-chefs-blood (Snippet: Whatever the animal, blood's deep, rich color is not lost on chefs ... sweet takeout pad Thai. (Ricker uses pig's blood instead of the ...))]
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Fictional Contexts: In fantasy settings like Dungeons & Dragons, the concept of "sweet blood" might be introduced as a fictional element. [Reference: Which sentient races have the tastiest blood? : r/DnD https://www.reddit.com/r/DnD/comments/dd7of3/which_sentient_races_have_the_tastiest_blood/ (Snippet: 4 out of 5 people who took that taste test said bugbear blood is sweet and tasty. ... Includes animal blood.)]
Therefore, the premise of the question requires clarification. No animal possesses naturally sweet blood. Sweetness is added artificially in culinary contexts or is entirely fictional.