The Keep:Valley of Riddles

Valley of Riddles is a guild in HabitRPG. Actually, it is one of the Small Guilds of Habitica, according to the Almost Complete Guide to the Guilds of Habitica. You can visit and join it using the direct link.

Guild Description
The guild helps and motivates its members to find, create and share nice riddles, puzzles, brain twisters and mind experiments, which exercise brain and give food for thoughts.

What about the cactus?
by Sphinx Cow - 3, Dogs - 4, Frog - 6, Rooster - 14, Cactus - ?

Cow says MOO (3 letters), Dogs say WOOF (4 letters), Frog says RIBBIT (6 letters), Rooster says COCK-A-DOODLE-DOO (14 letters excluding dashes), and Cactus is silent (0 letters). So the answer is 0.

What do the pets do, that can have a countable attribute?

Leave cactus alone for a bit. All others are clearly pets. What can they have in common?