I will list all valid beasts below, ordered by their appearance in each book and sorted by source.
Creatures with fly speed or swim speed (none of which are forbidden by the description in the Artificer class) will be marked accordingly. Creatures with other noteworthy traits (such as being unable to breathe outside of water or possessing an innate climb speed) will have a note attached.
- Allosaurus CR2, p. 79
- Plesiosaurus CR2, p. 80
- Axe Beak, CR1/4, p. 317
- Brown Bear, CR1, p. 319 (climb speed)
- Camel, CR1/8, p. 320
- Constrictor Snake, CR1/4, p. 320
- Crocodile, CR1/2, p. 320
- Dire Wolf, CR1, p. 321
- Draft Horse, CR1/4, p. 321
- Elk, CR1/4, p. 322
- Giant Bat, CR1/4, p. 323
- Giant Boar, CR2, p. 323
- Giant Eagle, CR1, p. 324
- Giant Goat, CR1/2, p. 326
- Giant Hyena, CR1, p. 326
- Giant Lizard, CR1/4, p. 326 (climb speed)
- Giant Octopus, CR1, p. 326 (can only breathe underwater)
- Giant Owl, CR1/4, p. 327
- Giant Sea Horse, CR1/2, p. 328 (can only breathe underwater, no walking speed)
- Giant Spider, CR1, p. 328 (climb speed (including ceilings))
- Giant Toad, CR1, p. 329
- Giant Vulture, CR1, p. 329
- Hunter Shark, CR2, p. 330 (can only breathe underwater, no walking speed)
- Lion, CR1, p. 331
- Polar Bear, CR2, p. 334
- Rhinoceros, CR2, p. 336
- Riding Horse, CR1/4, p. 336
- Saber-Toothed Tiger, CR2, p. 336
- Tiger, CR1, p. 339
- Warhorse, CR1/2, p. 340
Volo's Guide To Monsters
- Hadrosaurus, CR1/4, p. 140
- Aurochs, CR2, p. 207
- Cow, CR1/4, p. 207
Mordenkainen's Tome of Foes
There are no beasts in this book.
Tales from the Yawning Portal
Giant Crayfish CR2, p.235
Storm King's Thunder
Crag Cat, CR1, p.240 (spell turning, nondetection, lowish stats for CR 2, stealth expertise, special crawford tweet)
Apparently, some of the Adventurer's League books feature their own, exclusive creatures. I don't have any of those books - anyone who does, feel free to add them to this list.
The following is a somewhat subjective rating. You might want to use a creature not listed below if you're looking for specific traits or features.
Aside from that, these are what I consider the strongest or most versatile creatures (top 5 are emphasized):
- Plesiosaurus: Highest HP, decent damage, land+swim speed. Not sure why it has a walking speed, that makes it somewhat cheesy to choose.
- Polar Bear: decent stats with the highest damage; land creature with swim speed
- Hunter Shark (water only! 30ft blindsight, adv. on hurt creatures)
- Brown Bear: only relevant if you want climb speed, has otherwise inferior stats
- Giant Spider: even worse stats than the Brown Bear stats, but can climb ceilings, has Web and blindsight
- Giant Eagle or Giant Vulture: the latter has Pack Tactics and better senses, the eagle has otherwise better stats, including a higher fly speed
- Allosaurus: Among the highest HP, decent damage, highest land speed.
- Saber-Toothed Tiger: Among the highest HP, average damage, has Keen Smell and Pounce (DC14)
- Crag Cat: Immunity to magical detection plus stealth expertise plus spell turning make this by far the best option for characters looking to sneak around with their hulking mechanical servitor (or have their hulking mechanical servitor sneak around for them), especially at higher levels. Be aware the spell reflection applies even to spells you cast to buff it, where applicable, and that this creature's (pretending to be) only CR1 so it can't keep up in combat with e.g. the Allosaurus.